[docs] Document admin role, suggest deletion, and delete suggestions features

[neeraj-pilot]

Summary

• Document the new Admin role for album sharing with permissions between Collaborator and Owner
• Document the Suggest Deletion feature for owners/admins to suggest photo deletions
• Document the Delete Suggestions review flow under Backup > Free up space

Changes

Features documented:

• Admin role: Elevated permissions to manage participants and remove any photo, but cannot manage link settings
• Suggest deletion: Owners/admins can suggest deletion of photos they don't own (mobile only)
• Delete suggestions: Photo owners can review suggestions under Settings > Backup > Free up space (mobile only)

Files updated:

• collaboration.md - Admin permissions, use cases, suggest deletion section
• deleting.md - Role-based deletion permissions, suggest deletion feature
• storage-optimization.md - Delete suggestions review section
• sharing-and-collaboration.md (FAQ) - 9 new FAQs for admin and suggest deletion
• albums-and-organization.md (FAQ) - Updated collaborative deletion FAQ

Test plan

• Verify docs build successfully with yarn build
• Review new sections render correctly
• Check all internal links work
• Verify FAQ anchor IDs are unique