Scrub Restic references from main Velero docs

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Please add a summary of your change

This PR removes Restic-specific content from site/content/docs/main and aligns File System Backup docs with current Kopia-only guidance. It also updates related pages so terminology, examples, and maintenance guidance are internally consistent.

• File System Backup docs

  • Removed Restic deprecation/integration/repository sections from file-system-backup.md.
  • Updated intro and references to describe Kopia-only data movement.

• Related doc consistency updates

  • backup-repository-configuration.md: removed Restic from repository type wording.
  • repository-maintenance.md: removed Restic-specific type/value/example fields and default frequency text.
  • self-signed-certificates.md: updated TLS note to Kopia-only behavior.
  • performance-guidance.md: removed Restic comparisons and rewrote conclusions to standalone Kopia guidance.

Example snippet updated in docs:

spec:
  backupStorageLocation: default
  maintenanceFrequency: 1h0m0s
  repositoryType: kopia
  volumeNamespace: test

Does your change fix a particular issue?

Removes Restic references from main documentation content per the issue request and parent cleanup scope.

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• Updated the corresponding documentation in site/content/docs/main.

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