Document and communicate repo owner's guide to source-build for 7.0

[MichaelSimons]

The source-build team needs a repo owner's guide to source-build. This should cover the following:

1. How to build source-build - both at the repo level and the product level.
2. Prebuilts
   1. What considerations need to be taken when adding new dependencies during feature design/development
   2. How do detect and remove. This should discuss tooling that exists now and the desired UX.
3. PVP flow
   1. Discuss current flow and how source-build always flows the latest as well as the desired UX.
4. Patches
5. Repo owners' source-build responsibilities
6. CI
7. Contacts

Once completed, this should be rolled out/communicated to the source-build champs as well as to .NET tactics.