Adjust for backup restore doc moves

What does this MR do and why?

Describe why, not just what. Include links to relevant issues or discussions.

What:

Move back up docs from doc/raketasks/ to doc/administration/backup_restore/ as part of gitlab!123992 (merged).

Why:

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

Part of gitlab#396346 (closed)

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