Overall, the ladder structure remains the same, moving from a community
member, to a project participant, to a project leader. The goal of these
proposed changes is to encourage more community participation.

The primary difference is the change from a Triage role to a Reviewer
role. "Triage" did not align well with GitHub permissions, and could be
interpreted as granting the ability to review PRs without any control. I
have clarified that the now-called "Reviewer" role should be granted the
permission to approve a PR for merge, but not have the permission to
merge it. This should hopefully encourage community members who are
active in the codebase but not yet familiar with the entire project to
participate more and become more familiar through code reviews.

I have also clarified the requirements of a codeowner and reviewer to
emphasize their difference. A reviewer should have knowledge of the code
but not necessarily a complete understanding of the codebase and its
intricacies, while a codeowner should have a holistic understanding to
determine when there are backward compatibility issues, security
concerns, API breakage, etc.

With these changes, we can also move towards an entirely GH-team-based
permission model, with "reviewer" and "codeowner" teams per-repo.

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>