Move Mattermost doc from Omnibus to main project

What does this MR do?

Move docs from Omnibus project to main project.

related MRs:

• !70516 (merged)
• gitlab-docs!2140 (merged)
• omnibus-gitlab!5610 (merged)

part of "Move all the Omnibus documentation to the gitlab-org/gitlab project" &5173

Related issues

omnibus-gitlab#6110 (closed)

Moving docs to a new location?

Read the guidelines: https://docs.gitlab.com/ee/development/documentation/index.html#move-or-rename-a-page

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ and ee/app/views (for GitLab EE) directories.
• Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
• Update the link in features.yml (if applicable)
• Assign one of the technical writers for review.