Extend upgrade guide with backup instructions

docs/setup/upgrade.md

@@ -1,10 +1,50 @@
 # Upgrade an existing installation
 
-Update an existing headscale installation to a new version:
+!!! tip "Recommended update path"
+
+    Its best to update from one stable version to the next (e.g. 0.26.0 → 0.27.1 → 0.28.0) in case you are multiple
+    releases behind. You should always pick the latest available patch release.
+
+Update an existing Headscale installation to a new version:
 
 - Read the announcement on the [GitHub releases](https://github.com/juanfont/headscale/releases) page for the new
-  version. It lists the changes of the release along with possible breaking changes.
-- **Create a backup of your database.**
-- Update headscale to the new version, preferably by following the same installation method.
+  version. It lists the changes of the release along with possible breaking changes and version-specific upgrade
+  instructions.
+- Stop Headscale
+- **[Create a backup of your installation](#backup)**
+- Update Headscale to the new version, preferably by following the same installation method.
 - Compare and update the [configuration](../ref/configuration.md) file.
-- Restart headscale.
+- Start Headscale
+
+## Backup
+
+Headscale applies database migrations during upgrades and we highly recommend to create a backup of your database before
+upgrading. A full backup of Headscale depends on your individual setup, but below are some typical setup scenarios.
+
+=== "Standard installation"
+
+    A installation that follows our [official releases](install/official.md) setup guide uses the following paths:
+
+    - [Configuration file](../ref/configuration.md): `/etc/headscale/config.yaml`
+    - Data directory: `/var/lib/headscale`
+    - SQLite as database: `/var/lib/headscale/db.sqlite`
+
+    ```console
+    TIMESTAMP=$(date +%Y%m%d%H%M%S)
+    cp -aR /etc/headscale /etc/headscale.backup-$TIMESTAMP
+    cp -aR /var/lib/headscale /var/lib/headscale.backup-$TIMESTAMP
+    ```
+
+=== "Container"
+
+    A installation that follows our [container](install/container.md) setup guide uses a single source volume directory
+    that contains the configuration file, data directory and the SQLite database.
+
+    ```console
+    cp -aR /path/to/headscale /path/to/headscale.backup-$(date +%Y%m%d%H%M%S)
+    ```
+
+=== "PostgreSQL"
+
+    Please follow PostgreSQL's [Backup and Restore](https://www.postgresql.org/docs/current/backup.html) documentation
+    to create a backup of your PostgreSQL database.