Move linting rules to this repo

We have an inconsistent set of linting rules for Markdown content used on each of the sites:

- https://github.com/GoogleChrome/web.dev/tree/main/tools/linting
- https://github.com/GoogleChrome/developer.chrome.com/tree/main/tools/linting

We should evaluate each rule and unless there's a strong reason not to, move each to this repo for consistency's sake.