Merge branch 'docs/instance_template_repository_ssot' into 'master'

Edit *Instance template repository* for SSOT epic

See merge request gitlab-org/gitlab-ce!29032

doc/user/admin_area/settings/instance_template_repository.md

+---
+type: reference
+---
+
 # Instance template repository **[PREMIUM ONLY]**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/5986) in
@@ -61,3 +65,15 @@ top of the list.
 
 If this feature is disabled or no templates are present, there will be
 no "Custom" section in the selection dropdown.
+
+<!-- ## Troubleshooting
+
+Include any troubleshooting steps that you can foresee. If you know beforehand what issues
+one might have when setting this up, or when something is changed, or on upgrading, it's
+important to describe those, too. Think of things that may go wrong and include them here.
+This is important to minimize requests for support, and to avoid doc comments with
+questions that you know someone might ask.
+
+Each scenario can be a third-level heading, e.g. `### Getting error message X`.
+If you have none to add when creating a doc, leave this section in place
+but commented out to help encourage others to add to it in the future. -->