[Help Wanted] Document best practice for GitLab-specific CLI tool configuration to GitLab Mattermost Admin Guide

While GitLab omnibus is set up to manage Mattermost via the gitlab.rb file and a custom layout of Mattermost files, some community members want more control by using the Mattermost CLI tool, which requires additional configuration steps because of the GitLab omnibus layout.

This request is to ask help from the community to make a merge request to document the additional steps in the GitLab Mattermost Admin Guide, which can be verified by the GitLab omnibus team and included in a future release.

This issue is cross-posted to the GitLab Mattermost forum