Docs: guide on how to enable/disable feature flags for GL instances admins

Problem to solve

We have deployed a lot of features behind feature flags in GitLab. We should document how a GitLab self-managed instance admin should enable and disable features behind feature flags, explain the risks, and the advantages of doing so. We could prob add what to do in case they find problems with the features, how they can give us feedback so we can fix stuff based on real feedback.

Who can address the issue

~"devops::configure" team, I guess? 🤔 @eread, I'll assign to you, but if it's not exactly ~"devops::configure" let's see what we do.

cc/ @axil as he wrote docs on feature flags recently.

Other links/references

The conversation began here: gitlab-com/www-gitlab-com!34641 (diffs, comment 246314601)

I've seen other folks asking about this too.

Besides, we can concentrate the generic information in one doc and use it as a reference in other docs documenting features behind feature flags.