Improve Documentation

[DhruvaG2000]

The Docs folder has already been created in #20 but it is mostly written in markdown.

In future it may be better to shift to Sphinx as it will look more professional and organised. Github also can build these docs and publish them on a website. See sphinx: github pages

[DhruvaG2000]

TODOs:

• Document overlay changes needed to have PWM as in zephyrCommon: Implement analogWrite() #56
• Document overlay changes needed to have analog read functional in new variants zephyrCommon: Implement analogRead()/analogReference()  #54

[soburi]

PWM/ADC documentation was done in #66.