Resolve "[Feature flag] Rollout of `markdown_include_directive`"

See merge request !174584



Merged-by: Michael Becker <mbecker@gitlab.com>
Approved-by: Sarah Yasonik <syasonik@gitlab.com>
Approved-by: Michael Becker <mbecker@gitlab.com>
Co-authored-by: Brett Walker <bwalker@gitlab.com>