Move Mattermost doc from Omnibus to main project

Move docs from Omnibus project to main project.

related MRs:

• gitlab!70516 (merged)
• !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)

• Link to the merge request of the new page:

• Give the MR a descriptive title.

• Link to the merge request that introduced the new page.