doc: update repo perms docs for caching and sync speed (!24329) · Merge requests · Warren Gifford / sourcegraph · GitLab

Warren Gifford requested to merge doc/perms-caching into main Aug 25, 2021

Created by: bobheadxi

consolidate sync speed and mitigation docs
consolidate github docs
add docs for graphql endpoint for perms cache bypass
mark perms caching as experimental

follow-up https://github.com/sourcegraph/sourcegraph/pull/23978