Add global navigation item for new backup topic

What does this MR do and why?

As a follow up to: gitlab!125182 (merged), add a new global navigation item for the new docs file.

Screenshots, screen recordings, or links to review app

Review app: https://eread-add-new-backup-topic-to-global-nav.docs.gitlab-review.app/ee/administration/backup_restore/backup_large_reference_architectures.html

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.