Related: https://github.com/github/accessibility/issues/2542
This PR enables markdown linting pulling in configs from markdownlint-github.
Why? This repo is primarily composed of markdown files. There are valuable markdownlint rules that encourage best accessibility practices. We have config recs centralized in markdownlint-github now that should be pulled in.
markdownlint-github configs enables the markdownlint defaults, but then adds some GitHub custom rules and overrides. Some of markdownlint's defaults are related to stylistic things like line-length and blanks-around-headings. I don't know how much value Primer gets in enforcing these stylistic things so I decided to turn them off. There's also a lot to resolve, but if you think they should be enabled, it should probably be done in a follow-up.
I made sure the rules pertaining to accessibility are kept on since these provide a lot of value and encourage accessibility practices. I think those should be addressed as part of this PR. Open to reviewer opinions!