Remove content from FAQ and distribute to other locations

What does this MR do?

Contributes to: #347257.

Brings FAQ information into section of the documentation where it is most relevant.

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• Before: https://docs.gitlab.com/ee/administration/gitaly/faq.html#how-does-gitaly-cluster-compare-to-geo
• After: http://main-ee-88364.35.193.151.162.nip.io/ee/administration/gitaly/index.html#comparison-to-geo

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• Before: https://docs.gitlab.com/ee/administration/gitaly/faq.html#are-there-instructions-for-migrating-to-gitaly-cluster
• After: Removed, because was just a cross link.

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• Before: https://docs.gitlab.com/ee/administration/gitaly/faq.html#what-are-some-repository-storage-recommendations
• After: http://main-ee-88364.35.193.151.162.nip.io/ee/administration/gitaly/praefect.html#repository-storage-recommendations

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• Before: https://docs.gitlab.com/ee/administration/gitaly/faq.html#what-are-some-praefect-database-storage-requirements
• After: http://main-ee-88364.35.193.151.162.nip.io/ee/administration/gitaly/praefect.html#running-gitlab-and-praefect-database-on-same-server

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• Before: https://docs.gitlab.com/ee/administration/gitaly/faq.html#is-pgbouncer-required-for-the-praefect-database
• After: Rolled into http://main-ee-88364.35.193.151.162.nip.io/ee/administration/gitaly/praefect.html#use-pgbouncer

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• Before: https://docs.gitlab.com/ee/administration/gitaly/faq.html#are-there-any-special-considerations-for-gitaly-cluster-when-postgresql-is-upgraded
• After: http://main-ee-88364.35.193.151.162.nip.io/ee/administration/gitaly/praefect.html#requirements

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• Before: https://docs.gitlab.com/ee/administration/gitaly/faq.html#praefect-database-tables-are-empty
• After: Rolled into http://main-ee-88364.35.193.151.162.nip.io/ee/administration/gitaly/praefect.html#postgresql

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Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
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