This is a follow up to rails#47186, this time for all markdown content.

[markdownlint](https://github.com/markdownlint/markdownlint) is an excellent tool, and I've found it very useful for finding issues in the guides.

Many of the rules are common style issues I'm correcting on PRs, so it will be nice to have that automated.

We should also be able to use the same config with our editors, so that errors show up in real-time 🙏 and will update the contributing docs once this gets merged with how to debug and use mdl appropriately.