docs/library/machine.*: Add cross-reference label to individual classes.

docs/library/machine.ADC.rst

 .. currentmodule:: machine
+.. _machine.ADC:
 
 class ADC -- analog to digital conversion
 =========================================

docs/library/machine.I2C.rst

 .. currentmodule:: machine
+.. _machine.I2C:
 
 class I2C -- a two-wire serial protocol
 =======================================

docs/library/machine.Pin.rst

 .. currentmodule:: machine
+.. _machine.Pin:
 
 class Pin -- control I/O pins
 =============================

docs/library/machine.RTC.rst

 .. currentmodule:: machine
+.. _machine.RTC:
 
 class RTC -- real time clock
 ============================

docs/library/machine.SD.rst

 .. currentmodule:: machine
+.. _machine.SD:
 
 class SD -- secure digital memory card
 ======================================

docs/library/machine.SPI.rst

 .. currentmodule:: machine
+.. _machine.SPI:
 
 class SPI -- a Serial Peripheral Interface bus protocol (master side)
 =====================================================================

docs/library/machine.Timer.rst

 .. currentmodule:: machine
+.. _machine.Timer:
 
 class Timer -- control hardware timers
 ======================================

docs/library/machine.UART.rst

 .. currentmodule:: machine
+.. _machine.UART:
 
 class UART -- duplex serial communication bus
 =============================================

docs/library/machine.WDT.rst

 .. currentmodule:: machine
+.. _machine.WDT:
 
 class WDT -- watchdog timer
 ===========================