Netdata Agent: Backup restore documentation

[luisj1983]

Summary

A document on how to backup and restore the Netdata agent for the public docs. This works in a test environment and I've used it once in production so please check it over as it may need ammendments.

For users: How does this change affect me?

- Public-facing documentation change

[tkatsoulas]

@luisj1983 Added your current change in learn's stagging to have an idea on how it will look https://staging1--netdata-docusaurus.netlify.app/docs/maintaining/backup-netdata-agent-s-data

If you want to test your changes locally

1. Apply any change in your remote branch
2. Clone netdata learn https://github.com/netdata/learn
3. Checkout to staging1 branch (you don't have permission to push on the remote branch)
4. Follow https://github.com/netdata/learn?tab=readme-ov-file#prerequisites
5. Fetch new changes
   1. python ingest/ingest.py --repos luisj1983/netdata:maintenance_backups
   2. python ingest/create_grid_integration_pages.py
6. run yarn start