Add basic A2DP BluetoothAudio documentation (#2217)

docs/a2dp.rst

+Bluetooth Audio (A2DP Source and Sink)
+======================================
+
+The PicoW can be used as a Bluetooth Audio sink or source with the ``BluetoothAudio`` class.
+Operation is generally handled "automatically" in the background so while the audio is
+playing or streaming the main application can perform other operations (like displaying
+playback info, polling buttons for controls, etc.)
+
+.. code :: cpp
+
+    #include <BluetoothAudio.h>
+    ...
+
+**Note about CPU usage:**  Bluetooth SBC audio is a compressed format.  That means
+that it takes non-trivial amounts of CPU to compress on send, or decompress on receive.
+Transmitting precompressed audio from, say, MP3 or AAC, requires first decompressing
+the source file into raw PCM and then re-compressing them in the SBC format.  You may
+want to consider overclocking in this case to avoid underflow.
+
+A2DPSink
+--------
+
+This class implements slave sink-mode operation with player control (play, pause, etc.) and
+can play the received and decoded SBC audio to ``PWMAudio``, ``I2S``, or a user-created
+`BluetoothAudioConsumer`` class.
+
+The ``A2DPSink.ino`` example demonstrates turning a PicoW into a Bluetooth headset with
+``PWMAudio``.
+
+A2DPSource
+-----------
+
+This class implements a master source-mode SBC Bluetooth A2DP audio connection which
+transmits audio using the standard ``Stream`` interface (like ``I2S`` or ``PWMAudio``.
+The main application connects to a Bluetooth speaker and then writes samples into a buffer
+that's automatically transmitted behind the scenes.
+
+The ``A2DPSource.ino`` example shows how to connect to a Bluetooth speaker, transmit
+data, and respond to commands from the speaker.

docs/index.rst

@@ -46,6 +46,7 @@ For the latest version, always check https://github.com/earlephilhower/arduino-p
 
    Bluetooth <bluetooth>
    Bluetooth HID Master <hidmaster>
+   Bluetooth Audio (A2DP) <a2dp>
 
    Single File USB Drive <singlefile>
    FatFSUSB - full FS access over USB <fatfsusb>