[Help Wanted] Documenting gitlab.rb.template with Mattermost config settings

Currently GitLab Mattermost can only be upgraded using GitLab omnibus if IT Admins have configured Mattermost via changes to `gitlab.rb.template' exclusively (the alternative is to migrate Mattermost outside of omnibus control and manage separately).

However, gitlab.rb.template only includes a subset of Mattermost configuration options.

This ticket is asking the GitLab community for help documenting how GitLab users could properly update and maintain Mattermost settings in gitlab.rb.template via a merge request to the GitLab Mattermost Admin Guide.