Update docs and signposts on the monorepo to make sense

[mattwynne]

Now that all the code has been removed from the monorepo, we need to update the README to explain the purpose of the repo and give some signposts off to other places.

[mpkorstanje]

I can't find it in the docs right now but we can have a org level README on GitHub. It will show up prominently to people who visit the org.

[davidjgoss]

https://docs.github.com/en/organizations/collaborating-with-groups-in-organizations/customizing-your-organizations-profile#adding-a-public-organization-profile-readme