Add branch protection for main branch

[rasmi]

Require pull requests with review approvals to merge changes into the main branch.

[bion]

I added branch protection to require PRs but would prefer we not require reviews for ease of updating developer docs

[rasmi]

I disabled it temporarily until we can figure out how to allow gitbook-bot to bypass the branch protection (it is useful for some team members to make edits from GitBook directly and right now there's no obvious way to do that with branch protection enabled). I will keep an eye out for vandalism in the meantime! But agreed, branch protection + no reviews for members seems like a good setup. We can also set up teams in the CiviForm GitHub org to better manage permissions.