docs: how-to add backups for new hosts

docs/backups.md

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+# Backups
+
+We use [Restic](https://restic.readthedocs.io/en/stable/) to create backups and push them to two repositories.
+Check `./modules/backups.nix` and `./hosts/nachtigall/backups.nix` for working examples.
+
+### Hetzner Storagebox
+
+- Uses SFTP for transfer of backups
+
+Adding a new host SSH public key to the storagebox:
+
+First, [SSH to nachtigall](./administrative-access.md#ssh-access), then become root and add the new SSH public key
+
+```
+sudo -i
+echo '<ssh-public-key>' | ssh -p23 u377325@u377325.your-storagebox.de install-ssh-key
+```
+
+[Link to Hetzner storagebox docs](https://docs.hetzner.com/robot/storage-box/backup-space-ssh-keys).
+
+### Garage S3 buckets
+
+- Uses S3 for transfer of backups
+- One bucket per host, e.g. `nachtigall-backups`, `metronom-backups`
+
+To start transfering backups from a new hosts, this is how to create a new bucket:
+
+First, [SSH to trinkgenossin](./administrative-access.md#ssh-access), then use the `garage` CLI to create a new key and bucket:
+
+```
+export GARAGE_RPC_SECRET=<secret-in-keepass>
+
+garage bucket create <hostname>-backups
+garage key create <hostname>-backups-key
+garage bucket allow <hostname>-backups --read --write --key <hostname>-backups-key
+```