Linking to backup doc from migration doc, for self-hosted to self-hosted migration

What does this MR do?

It's not clear in documentation that users migrating between two self-hosted servers can simply backup/restore, so this adds a small section to the migration doc linking to the backup/restore doc.

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56339

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards