Add a note about feature not being available yet

- Since the self-monitoring project was not released in the planned
milestone, but the documentation did get released, update the docs now
to note that the feature is not yet available.

doc/administration/monitoring/gitlab_instance_administration_project/index.md

 # GitLab instance administration project
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/issues/56883) in GitLab 12.2.
+NOTE: **Note:**
+This feature is not yet available and is [planned for 12.6](https://gitlab.com/gitlab-org/gitlab/issues/32351).
 
 GitLab has been adding the ability for administrators to see insights into the health of
 their GitLab instance. In order to surface this experience in a native way, similar to how