Expand wiki as a knowledge base for Backrest

[garethgeorge]

This is a high level tracking bug for articles I'd like to add to the wiki

• [ ] SSH target configuration, likely based on the nice guide in discussions contributed by @Glideh https://github.com/garethgeorge/backrest/discussions/341
• [ ] Restoring backrest backups on a new host.
• [x] Update documentation re: backrest's operational details
• [ ] Windows specific guides re: new installers (coming in next release)
• [x] Security e.g. password management, securing public access, etc.

Feel free to request guides in this thread or link discussions that deserve adding to the wiki.