docs/library: Add missing cross-ref links for classes in pyb module.

docs/library/pyb.Accel.rst

 .. currentmodule:: pyb
+.. _pyb.Accel:
 
 class Accel -- accelerometer control
 ====================================

docs/library/pyb.CAN.rst

 .. currentmodule:: pyb
+.. _pyb.CAN:
 
 class CAN -- controller area network communication bus
 ======================================================

docs/library/pyb.LCD.rst

 .. currentmodule:: pyb
+.. _pyb.LCD:
 
 class LCD -- LCD control for the LCD touch-sensor pyskin
 ========================================================

docs/library/pyb.Switch.rst

 .. currentmodule:: pyb
+.. _pyb.Switch:
 
 class Switch -- switch object
 =============================

docs/library/pyb.USB_HID.rst

 .. currentmodule:: pyb
+.. _pyb.USB_HID:
 
 class USB_HID -- USB Human Interface Device (HID)
 =================================================

docs/library/pyb.USB_VCP.rst

 .. currentmodule:: pyb
+.. _pyb.USB_VCP:
 
 class USB_VCP -- USB virtual comm port
 ======================================