Convert docs, READMEs, and changenotes to Markdown

[kattni]

What is the problem or limitation you are having?

Docs, utils/README.rst and changenotes are still formatted in reStructuredText.

Describe the solution you'd like

Convert all rST files to Markdown.

Describe alternatives you've considered

Additional context

This will bring colosseum in line with the rest of the BeeWare projects and documentation.