proj: review settings

[silopolis]

• proj: set review requirements on 'main' branch
• proj: set auto-merging behavior
• proj: configure notifications
• proj: configure auto-assignment
• docs: proj: write project review settings and policy docs

References:

• https://docs.github.com/en/organizations/organizing-members-into-teams/managing-code-review-settings-for-your-team
• https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests
• https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/requesting-a-pull-request-review
• https://docs.github.com/en/contributing/collaborating-on-github-docs/self-review-checklist
• https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-stage-of-a-pull-request