Update feature flag documentation
After &8005 (closed) yields some early results, the way we use feature flag changes. Now Gitaly fully supports feature flag actors. Although the set of supported actors are the same, the way we intercept them is a bit different from GitLab Rails
This commit updates the feature flag documentation:
- Add overall architecture and explains how a flag is propagated around
- Add FF Development guideline
- Add new rollout strategies
Doc preview link: https://gitlab.com/gitlab-org/gitaly/-/blob/qmnguyen0711/feature-flag-actors-update/doc/PROCESS.md
Edited by Quang-Minh Nguyen