Improve Lawnchair documentation

[JovannMC]

Description

Opened on behalf of @validcube.

This PR aims to improve the documentation of Lawnchair by creating a CONTRIBUTING.md and modifying the existing markdown files.

Type of change

:white_check_mark: General change (non-breaking change that doesn't fit the below categories like copyediting) :x: Bug fix (non-breaking change which fixes an issue) :x: New feature (non-breaking change which adds functionality) :x: Breaking change (fix or feature that would cause existing functionality to not work as expected)