Docs update for updating Omnibus

Closes: gitlab-com/support/support-team-meta#2308 (closed)

What does this MR do?

The MR updates the Omnibus Update documentation by making clarifications and improvments to help reduce Support tickets as mentioned in this issue.

Related issues

gitlab-com/support/support-team-meta#2308 (closed)

Changing the header title?

• Is it completely necessary to change the title? Changing titles breaks deep linking. If yes, proceed further.
• Move the old heading title to the bottom of the page.
• Change the header tag to h5, #####.

Moving docs to a new location?

• 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.