Reorganize back up and restore docs

What does this MR do?

• Reorganizes back up and restore docs because it is very confusing.
  • Move back up docs from doc/raketasks/ to doc/administration/backup_restore/ because:
    1. the back up command is no longer just a rake task
    2. much of the docs discuss more than just the back up command
    3. we need a top-level, general backup/restore document anyway
  • Move back-up-specific sections from the top-level doc to the "Back up GitLab" doc
• Explain more about what the backup command is trying to accomplish, to help users extrapolate the docs to apply to their specific use case.

MR to adjust docs.gitlab.com nav links: gitlab-docs!3998 (closed)

Related issues

#396346 (closed)

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