Add documentation for Backup integration

[ludeeus]

Proposed change

Related:

• Add Backup integration core#66395
• Add panel to Backup integration frontend#11671
• Add backup icon brands#3157
• Add documentation for Backup integration #21612

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logo's and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.

Additional information

• Link to parent pull request in the codebase: Add Backup integration core#66395
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue: fixes #

Checklist

• This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• The documentation follows the Home Assistant documentation standards.

[balloob]

Wonder if we can add a My link that works for both hassio + backup integration.

[ludeeus]

the frontend handles that, so yes

[ludeeus]

This route to the documentation not to the instance so not sure it should be here at all. but for other places, we can do it.