doc: Add maintenance mode notice
What does this MR do?
Add maintenance mode notice to common describing current state and extracted modules as covered with the work in gitlab-org/gitlab#211819 (closed).
What are the relevant issue numbers?
gitlab-org/gitlab#211819 (comment 532691583)
Does this MR meet the acceptance criteria?
- [-] Changelog entry added
- [-] Documentation created/updated for GitLab EE, if necessary
-
Documentation created/updated for this project, if necessary -
Documentation reviewed by technical writer or follow-up review issue created -
Tests added for this feature/bug -
Job definition updated, if necessary -
Conforms to the code review guidelines -
Conforms to the Go guidelines -
Security reports checked/validated by reviewer