Extend documentation with section on how to backup renterd

[peterjan]

This PR adds a section to our documentation explaining how to backup a renterd instance and restore from a backup. I've followed it for a MySQL instance and aside from some partial slabs that I forgot to include in my backup, I managed to successfully restore it.

[peterjan]

@ChrisSchinnerl this is an initial DRAFT of a new section in our README that deals with backup up renterd - I was originally thinking to include a script too but I'm not too sure now since a lot of it depends on the user's configuration... I've tested the above on local testnet nodes, both SQLite and MySQL and did not ran into any issues. I'm going to try it on my personal node next (which is a MySQL node that manages ~5TB). if that goes without issues I'd consider it a reasonable first version? Is there anything missing that you were thinking of? Let me know what you think!