[skip changelog] Document Arduino CLI's configuration methods (#748)

The ability to configure Arduino CLI via environment variables is only documented by a mention in the integration options article, which is not an obvious place to look for that information. Since this information didn't fit well in any of the existing pages, it makes sense to add a dedicated page, and add some documentation for the other two configuration methods as well. Although there is existing documentation for the other configuration methods elsewhere in the docs, it is missing some useful specifics, which wouldn't be appropriate to add there.

docs/configuration.md

+## Configuration keys
+
+* `board_manager`
+    * `additional_urls` - the URLs to any additional Board Manager package index
+    files needed for your boards platforms.
+* `daemon` - options related to running Arduino CLI as a [gRPC] server.
+    * `port` - TCP port used for gRPC client connections.
+* `directories` - directories used by Arduino CLI.
+    * `data` - directory used to store Board/Library Manager index files and
+    Board Manager platform installations.
+    * `downloads` - directory used to stage downloaded archives during
+    Board/Library Manager installations.
+    * `user` - the equivalent of the Arduino IDE's
+    ["sketchbook" directory][sketchbook directory]. Library Manager
+    installations are made to the `libraries` subdirectory of the user
+    directory.
+* `logging` - configuration options for Arduino CLI's logs.
+    * `file` - path to the file where logs will be written.
+    * `format` - output format for the logs. Allowed values are `text` or
+    `json`.
+    * `level` - messages with this level and above will be logged. Valid levels
+    are: `trace`, `debug`, `info`, `warn`, `error`, `fatal`, `panic`.
+* `telemetry` - settings related to the collection of data used for continued
+improvement of Arduino CLI.
+    * `addr` - TCP port used for telemetry communication.
+    * `enabled` - controls the use of telemetry.
+
+## Configuration methods
+
+Arduino CLI may be configured in three ways:
+
+1. Command line flags
+1. Environment variables
+1. Configuration file
+
+If a configuration option is configured by multiple methods, the value set by
+the method highest on the above list overwrites the ones below it.
+
+If a configuration option is not set, Arduino CLI uses a default value.
+
+[`arduino-cli config dump`][arduino-cli config dump] displays the current
+configuration values.
+
+### Command line flags
+
+Arduino CLI's command line flags are documented in the command line help and the
+[Arduino CLI command reference].
+
+#### Example
+
+Setting an additional Board Manager URL using the
+[`--additional-urls`][arduino-cli global flags] command line flag:
+
+```shell
+$ arduino-cli core update-index --additional-urls https://downloads.arduino.cc/packages/package_staging_index.json
+```
+
+### Environment variables
+
+All configuration options can be set via environment variables. The variable
+names start with `ARDUINO`, followed by the configuration key names, with each
+component separated by `_`. For example, the `ARDUINO_DIRECTORIES_USER`
+environment variable sets the `directories.user` configuration option.
+
+On Linux or macOS, you can use the [`export` command][export command] to set
+environment variables. On Windows cmd, you can use the
+[`set` command][set command].
+
+#### Example
+
+Setting an additional Board Manager URL using the
+`ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS` environment variable:
+
+```sh
+$ export ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS=https://downloads.arduino.cc/packages/package_staging_index.json
+```
+
+### Configuration file
+
+[`arduino-cli config init`][arduino-cli config init] creates or updates a
+configuration file with the current configuration settings.
+
+This allows saving the options set by command line flags or environment
+variables. For example:
+
+```sh
+arduino-cli config init --additional-urls https://downloads.arduino.cc/packages/package_staging_index.json
+```
+
+#### File name
+
+The configuration file must be named `arduino-cli`, with the appropriate file
+extension for the file's format.
+
+#### Supported formats
+
+`arduino-cli config init` creates a YAML file, however a variety of common
+formats are supported:
+
+* [JSON]
+* [TOML]
+* [YAML]
+* [Java properties file]
+* [HCL]
+* envfile
+* [INI]
+
+#### Locations
+
+Configuration files in the following locations are recognized by Arduino CLI:
+
+1. Location specified by the [`--config-file`][Arduino CLI command reference]
+command line flag
+1. Current working directory
+1. Any parent directory of the current working directory (more immediate parents
+having higher precedence)
+1. Arduino CLI data directory (as configured by `directories.data`)
+
+If multiple configuration files are present, the one highest on the above list
+is used. Configuration files are not combined.
+
+The location of the active configuration file can be determined by running the
+command:
+
+```sh
+arduino-cli config dump --verbose
+```
+
+#### Example
+
+Setting an additional Board Manager URL using a YAML format configuration file:
+
+```yaml
+board_manager:
+  additional_urls:
+  - https://downloads.arduino.cc/packages/package_staging_index.json
+```
+
+Doing the same using a TOML format file:
+
+```toml
+[board_manager]
+additional_urls = [ "https://downloads.arduino.cc/packages/package_staging_index.json" ]
+```
+
+
+[gRPC]: https://grpc.io
+[sketchbook directory]: sketch-specification.md#sketchbook
+[arduino-cli config dump]: ../commands/arduino-cli_config_dump
+[Arduino CLI command reference]: ../commands/arduino-cli
+[arduino-cli global flags]: ../commands/arduino-cli_config/#options-inherited-from-parent-commands
+[export command]: https://ss64.com/bash/export.html
+[set command]: https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/set_1
+[arduino-cli config init]: ../commands/arduino-cli_config_init
+[JSON]: https://www.json.org
+[TOML]: https://github.com/toml-lang/toml
+[YAML]: https://en.wikipedia.org/wiki/YAML
+[Java properties file]: https://en.wikipedia.org/wiki/.properties
+[HCL]: https://github.com/hashicorp/hcl
+[INI]: https://en.wikipedia.org/wiki/INI_file

docs/getting-started.md

@@ -53,7 +53,8 @@ Config file written: /home/luca/.arduino15/arduino-cli.yaml
 ```
 
 If you inspect the contents of `arduino-cli.yaml`, you'll find the available
-options with their respective default values.
+options with their respective default values. For more information, see the
+[configuration documentation].
 
 ## Create a new sketch
 
@@ -343,6 +344,7 @@ telemetry:
   addr: :9090
 ```
 
+[configuration documentation]: configuration.md
 [client_example]: https://github.com/arduino/arduino-cli/blob/master/client_example
 [gRPC reference]: ../rpc/commands
 [Prometheus]: https://prometheus.io/

docs/integration-options.md

@@ -41,7 +41,8 @@ package index that can be used to work with experimental versions of cores:
 One note about the example above: passing a value through a command line flag
 always takes precedence over reading an environment variable, which in turn
 always takes precedence over reading the same value from the configuration file
-(if you have one).
+(if you have one). For more information, see the [configuration documentation].
+
 Consistent with the previous paragraph, when it comes to providing output the
 Arduino CLI aims to be user friendly but also slightly verbose, something that
 doesn’t play well with robots. This is why we added an option to provide output
@@ -118,6 +119,7 @@ if you’ve got a use case that doesn’t fit one of the three pillars.
 [Arduino Create]: https://create.arduino.cc
 [continuous integration]: https://en.wikipedia.org/wiki/Continuous_integration
 [continuous deployment]: https://en.wikipedia.org/wiki/Continuous_deployment
+[configuration documentation]: configuration.md
 [JSON]: https://www.json.org
 [installation script]: installation.md#use-the-install-script
 [command reference]: ../commands/arduino-cli

mkdocs.yml

@@ -101,6 +101,7 @@ nav:
     - monitor: rpc/monitor.md
     - settings: rpc/settings.md
     - debug: rpc/debug.md
+  - configuration.md
   - Integration options: integration-options.md
   - sketch-build-process.md
   - sketch-specification.md