docs: Convert CONTRIBUTING .rst -> .md via rst2myst

[rht]

This has been brought up in dev meetings for a while, to have 1 format for documentation everywhere, not a mixture of .rst and .md. This is actually a long overdue change; @tpike3 could confirm this. The secondary reason is I want to try ChatGPT in making the docs clearer and more concise. See https://github.com/projectmesa/mesa/pull/2037.