Add RDB/AOF persistence for backups

[pshankinclarke]

✍️ Description

I enable RDB and AOF persistence in the same instance to make it as easy as possible for Proxmox backups to restore Valkey data. I confirmed that Debian already stores the RDB/AOF files at /var/lib/valkey. After the install I checked that appendonly and save were set correctly, and RDB + AOF persistence were enabled and writing. Finally I wrote a small dataset, ran a backup from the host, restored it into a new container, and confirmed the dataset was present.

🔗 Related PR / Issue

Link: # [discussion 3446] #3446

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected.
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.

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🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing functionality in a way that may require updates.
• 🆕 New script – A fully functional and tested script or script set.
• 🌍 Website update – Changes to website-related JSON files or metadata.
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, AppName.md, CONTRIBUTING.md, or other docs.

[MickLesk]

I think this should Not be a Part of the Default Script, better as Guide.