Add the MDN Code of Conduct, and refresh CONTRIBUTING for current practices:
* Our process is much closer to GitHub flow than the old WebDev workflow
* We have a lot more to say about code style, expecially around tests
* Add more guidance around branch names and commit messages
* Add more about the code review and deployment processes
* Add expectations for a PR to be touched in 2 business days
* Drop requirement for two peers to review database changes

Fix some issues that are easier to see when GitHub is rendering the
markdown, like long sentence, adding formatting, and fixing links,

[Foo](bar) links to ./bar, but [Foo][bar] uses the later reference [bar]