Moved main Geo page and setup instructions

What does this MR do?

This MR moves the general Geo overview up one level so that it is no longer specific to Replication. The reason is that this contains information that is relevant both to replication and DR.

In addition, I've moved the setup instructions to a separate sub-section in its own folder. This brings us closer to having clearly defined subsections for important tasks.

The global nav changes are in a related MR gitlab-docs!1135 (merged)

Related issues

Closes #243513 (closed)

Moving docs to a new location?

Read the guidelines: https://docs.gitlab.com/ee/development/documentation/index.html#changing-document-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.
• 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)
• If working on CE and the ee-compat-check jobs fails, submit an MR to EE with the changes as well (https://docs.gitlab.com/ce/development/documentation/index.html#cherry-picking-from-ce-to-ee).
• Ping one of the technical writers for review.