Add docs on v3 onion service restore-from-backup story

[rocodes]

Description

As we are encouraging the adoption of v3 onion services, we should improve on the documentation on restoring from a backup:

• with v3 only
• with v2+v3 (based on a current support portal request)

User Research Evidence

Open support ticket

User Stories

As an administrator, I want clear instructions on how to restore from a backup if my instance supports v3 onion services.

[eloquence]

To add a bit more context, the restore docs currently say:

This documentation applies to a SecureDrop instance using v2 onion services. If your instance uses v3 onion services, you will need to follow additional steps depending on your specific restore scenario.

Given that v3 is the recommended and default install method, we'll aim to make this less hand-wavey during the 9/17-10/1 sprint. Thanks much for filing and taking a stab at this, @rocodes!

[eloquence]

(Removed from board as #133, which is expected to fully resolve, is tracked there.)