[Process change] Limit the ability to merge into o3de.org:main to only sig-docs-community-maintainers

[chanmosq]

Purpose

Sig-docs-community and other admins or project maintainers should be the only ones who can merge pull requests into o3de.org:main.

What changed?

We changed the branch protection rules for o3de.org:main by enabling Restrict who can push to matching branches and specifying the sig-docs-community-maintainers team. We also adjusted the access-levels for the members of o3de.org repo.

The following teams have the ability to push to o3de.org:main:

• ProjectMaintainers
• sig-chairs
• sig-docs-community-maintainers