docs: extensive backup documentation

docs/site/guides/backups/advanced-example.md

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+This guide explains how to set up a [Hetzner Storage Box](https://docs.hetzner.com/storage/storage-box/general) as a backup destination instead of using an internal Clan backup server. Follow the steps below to configure and verify the setup.
+
+### Step 1: Create a Hetzner Storage Box
+
+Begin by [creating a Hetzner Storage Box account](https://docs.hetzner.com/storage/storage-box/getting-started/creating-a-storage-box).
+
+### Step 2: Create a Sub-Account
+
+Set up a sub-account for your `jon` machine. Save the SSH password for this account in your password manager for future reference.
+
+### Step 3: Configure BorgBackup in `clan.nix`
+
+Add the BorgBackup service to your `clan.nix` configuration. In this example, the `jon` machine will back up to `user-sub1@user-sub1.your-storagebox.de` in the `borgbackup` folder:
+
+```nix hl_lines="9"
+inventory.instances = {
+  borgbackup = {
+    module = {
+      name = "borgbackup";
+      input = "clan-core";
+    };
+    roles.client.machines."jon".settings = {
+      destinations."storagebox" = {
+        repo = "user-sub1@user-sub1.your-storagebox.de:/./borgbackup";
+        rsh = ''ssh -p 23 -oStrictHostKeyChecking=accept-new -i /run/secrets/vars/borgbackup/borgbackup.ssh'';
+      };
+    };
+  };
+};
+```
+
+### Step 4: Generate SSH Keys
+
+Run the following command to generate the SSH private keys:
+
+```bash
+clan vars generate
+```
+
+### Step 5: Add the Public Key to the Sub-Account
+
+Add the generated SSH public key to the `user-sub1` account by running:
+
+```bash
+clan vars get jon borgbackup/borgbackup.ssh.pub | ssh -p23 user-sub1@user-sub1.your-storagebox.de install-ssh-key
+```
+
+### Step 6: Deploy the Configuration
+
+Apply the changes to your Clan setup by executing:
+
+```bash
+clan machines update
+```
+
+### Step 7: Verify the Setup
+
+Check if the configuration works by starting the BorgBackup service on the `jon` machine:
+
+```bash
+systemctl start borgbackup-job-storagebox.service &
+```
+
+Then, inspect the service logs to ensure everything is functioning correctly:
+
+```bash
+journalctl -u borgbackup-job-storagebox.service
+```
+
+
+