Add How to restore from backup / create backup Nitrokey

File: [nitrokey3/linux/openpgp-keygen-backup.rst] https://docs.nitrokey.com/nitrokey3/linux/openpgp-keygen-backup.html

I'd like to suggest to add also some documentation how to restore from backup and how to install on a second backup nitrokey.

some points worth mentioning

• create backup nitrokey
  • how to switch between nitrokeys? how to update private-key stubs?
  • is it possible to "create" two nitrokeys one after the other, if during keytocard private-key stubs are not created (to replace the private key)?
  • is ADSK a valid strategy? Is there enough space on Nitrokey for this, according to documentation only 3 keys can be stored SC/E/A?
• in case of loss, install backup on new nitrokey
  • find the private-key stubs with --with-keygrip and delete
  • I had to use gpg --batch --import private.key, without batch I'd get sec#

Note:
this new page could be embedded in a series of pages related to how-to-recover-from-loss-or-damage. based on the forum posts, there appears to be a need for entry level documentation how to work with security tokens and the different strategies for backup and recovery (FIDO, PIV, secrets, PGP)

Start with nitrokey3/linux/openpgp-keygen-backup.rst and the linked ADSK documentation. Document creating and switching between backup Nitrokeys, restoring after loss, handling private-key stubs, and the listed import command behavior. Done means the backup and recovery workflows and the open capacity questions are clearly covered, possibly across a recovery documentation series.