docs: [#14] add comprehensive persistent volume documentation

- Add detailed comment explaining persistent data volume configuration
- Clarify that /dev/vdb is the second disk attached for persistent storage
- Document specific data types stored: database, config, SSL certs, logs, metrics
- Explain data survival across VM destruction and infrastructure recreation
- Improve maintainability and understanding of storage architecture

This makes the cloud-init configuration self-documenting for the persistent
data strategy implemented in the twelve-factor architecture.

Author: josecelano

infrastructure/cloud-init/user-data.yaml.tpl

@@ -33,7 +33,15 @@ users:
 # Disable SSH password authentication for security
 ssh_pwauth: false
 
-# Disk and filesystem configuration
+# Persistent data volume configuration
+# This configures the second disk (/dev/vdb) attached to the VM for persistent storage.
+# All application data that needs to survive VM destruction is stored here, including:
+# - Database data (MySQL)
+# - Application configuration files (.env, tracker.toml)
+# - SSL certificates and keys
+# - Logs and application state
+# - Prometheus metrics data
+# The volume is mounted at /var/lib/torrust and survives infrastructure recreation.
 disk_setup:
   /dev/vdb:
     table_type: gpt