Define markdown convention/formatting rules

It would be good to define our standards/expectations for markdown formatting somewhere consistently.

A good place for this would be `CONTRIBUTING.md`, but other places could work too.

In addition to introducing documentation on how markdown should be written in the repo, we should probably try and
implement some tooling that enables machines to check our output for us:

- https://github.com/hukkin/mdformat
- https://github.com/DavidAnson/markdownlint
- https://dprint.dev/overview/ w/ https://github.com/dprint/dprint-plugin-markdown
- Prettier (listed last because it's intentionally hard to configure)

I think a good PR that resolves this issue would have a comparison of the output of these tools on one of our files, along with integration of the tool into CI so that we can catch issues going forward.