Convert to Markdown

This is a proposal to convert the documentation to Markdown since many people are familiar with it (myself included) but are not familiar with ReStructuredText. Personally I don't have the time nor interest in learning another markup language when Markdown is the standard on GitHub. As a result, I have not been involved with the documentation lately.

https://hellosystem.github.io/docs/ uses Sphinx, recommonmark, and GitHub Actions to produce documentation from the Markdown source files in this repository: https://github.com/helloSystem/docs

Maybe we can use a similar tooling for this documentation?

NOTE: recommonmark is now deprecated. Its developers recommend using [MyST](https://github.com/executablebooks/MyST-Parser) for a docutils bridge going forward. See [this issue](https://github.com/readthedocs/recommonmark/issues/221) for background and discussion.