docs: Add links from quickref to pyb classes.

docs/library/pyb.ADC.rst

docs/library/pyb.DAC.rst

docs/library/pyb.ExtInt.rst

docs/library/pyb.I2C.rst

docs/library/pyb.LED.rst

docs/library/pyb.Pin.rst

docs/library/pyb.SPI.rst

docs/library/pyb.Timer.rst

docs/library/pyb.UART.rst

docs/quickref.rst