Draft: Update DR/Backups handbook pages

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Related to issue : gitlab-com/gl-infra/production-engineering#25788

• Added context to the main backup page from database disaster recovery
• Removed irrelevant links

1. Make several updates to the main backup page in the handbook
   • Cover both backups and restores
   • Cover both snapshot strategy and db backup strategy
   • Fold in the content from database disaster recovery handbook page to this page
   • Ensure the section on databases talks about all databases (or is not so specific), not just the main one
   • Add section for gamedays, referencing the gameday overview
2. Because the design doc for DR appears in google searches, add a header that links to the main backup page as the SSOT
3. Once (1) is complete remove the faq page as content on that page is like from 2019 and is outdated.
4. Once (1) is complete remove the database dr handbook page
5. Any place where we reference RTO/RPO should reference https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/disaster_recovery/ instead as the SSOT.

CHANGEME

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Commits

• Removed link to BCD

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