Expose time since starring on project/:id/starrers API endpoint; exclude private profiles here as well

Fix API endpoint for starred projects of a user; add info about starred projects on profile to documentation

Save instance administration project id in DB

See merge request gitlab-org/gitlab-ce!31111

- This will make it easy to identify the project even if admins change
the name of the project or move it.

Upgrade Gitaly to v1.57.0

See merge request gitlab-org/gitlab-ce!31568

Add Documentation for Feature Flag Percent Rollout

Closes gitlab-ee#8240

See merge request gitlab-org/gitlab-ce!29770

Add updated screen shots

Add Jenkins quick reference page

See merge request gitlab-org/gitlab-ce!31518