Added Geo multi-node HA instructions

What does this MR do?

This MR adds documentation for updating a multi-node / HA deployment that also uses Geo. This is based on an upgrade demo that @dbalexandre ran on 2019-09-03.

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7602 gitlab#33335 (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.