Add GerritForge Support Policy, SLO and SLA

Following the Issue 11170, raised during the last
Gerrit User Summit retrospective, and the discussion [1] at
the latest ESC meeting on the 1st of December, GerritForge has
committed to publish its Community and Enterprise Support
policies.

[1]: https://www.gerritcodereview.com/2020-12-01-esc-minutes.html#triaging-issues-follow-up-to-issue-11170-contributor-summit

Bug: Issue 11170
Change-Id: Ie06a035addc4db140ac426819f821f9b2fcb199b
1 file changed