Add code owners and CONTRIBUTING (PR #12294)
With code owners we can automatically request a code review from a code owner when a PR is no longer a draft.
Hopefully this helps contributors communicate they are ready for a review from core developers.
To facilitate this, new contributions should start in draft state, and be converted to "ready for review" when initial checks are complete.
This adds a code owners file, docs: https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners
Docs for auto-assign code reviewer: https://docs.github.com/en/organizations/organizing-members-into-teams/managing-code-review-assignment-for-your-team
Adds a CONTRIBUTING file which points to our full guide: https://github.com/nvaccess/nvda/wiki/Contributing
