docs/machine: Sort machine classes in logical order, not alphabetically.

The list starts with the simplest functionality - GPIO, proceeds to
communication interfaces (UART, SPI, I2C), the to time(r) related
things, then everything else.

docs/library/machine.rst

@@ -145,13 +145,13 @@ Classes
  .. toctree::
    :maxdepth: 1
 
-   machine.I2C.rst
    machine.Pin.rst
    machine.Signal.rst
-   machine.RTC.rst
+   machine.UART.rst
    machine.SPI.rst
+   machine.I2C.rst
+   machine.RTC.rst
    machine.Timer.rst
-   machine.UART.rst
    machine.WDT.rst
 
 .. only:: port_wipy
@@ -159,12 +159,12 @@ Classes
  .. toctree::
    :maxdepth: 1
 
-   machine.ADC.rst
-   machine.I2C.rst
    machine.Pin.rst
-   machine.RTC.rst
-   machine.SD.rst
+   machine.UART.rst
    machine.SPI.rst
+   machine.I2C.rst
+   machine.RTC.rst
    machine.Timer.rst
-   machine.UART.rst
    machine.WDT.rst
+   machine.ADC.rst
+   machine.SD.rst