Clarify Geo proxying docs

What does this MR do?

Clarify Geo proxying docs.

#436973 (closed)

• Help these users: It sounds like disabling Geo proxying should disable SSH proxying. But it doesn't!
• Direct users to upvote or comment with their use-cases for read-only sites.
• Answer this question: Is there is an alternative way to disable SSH proxying?

#388996 (closed)

• Make it clear early in the Geo docs that:

  Secondary Geo sites transparently behave like the primary, by default. Therefore Geo sites can be load balanced, with the caveat that you must use sticky sessions to avoid authentication failures and cross-site request errors.

• Search the docs for read-only and adjust for the fact that secondary proxying is enabled by default
• Remove https://docs.gitlab.com/ee/user/admin_area/geo_sites.html#multiple-secondary-sites-behind-a-load-balancer since it is now superseded by https://docs.gitlab.com/ee/administration/geo/secondary_proxy/location_aware_external_url.html

#437562 (closed)

• Incorporate this MR and its discussions: !148269 (closed)
• Fix this: We link to a separate page for URL configuration that I think can simply be merged into this page
• Search for links to location_aware_external_url in other projects and change them to index https://docs.gitlab.com/ee/development/documentation/redirects.html => gitlab-docs!4966 (merged) and gitlab-com/www-gitlab-com!135777 (merged)
• Fix this: The nomenclature is inconsistent
  • For instance, the Set up a unified URL for Geo sites and Geo proxying with Separate URLs should use more similar phrasing
• Incorporate this info: Javiera and I today tested "round-robin" Google Cloud DNS with a Geo environment and it worked fine. I think this is normally to be expected, since DNS is inherently sticky due to caching by the client as well as DNS caching in many possible places in-between.
  • If using a load-balancer, then routing must be sticky. Not round-robin.

#408102

• Reflow some the text to highlight proxying is the default and disabling it is an exception.
• Move limitations to a location that it easier more visible on the page.

Related issues

Resolves #436973 (closed)

Resolves #388996 (closed)

Resolves #437562 (closed)

Partially resolves #408102

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