[docs] Rework backups a bit

[daenney]

Description

This changes the existing backup documentation to:

• Push a bit harder on people to perform backups, it's not really just a nice to have
• Removes the language about migrating to/from GoToSocial and a different ActivityPub server since that's really not supported
• Adds a section about using backup software and provides an example on how to do this using Borgmatic

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[NyaaaWhatsUpDoc]

Looks great! A few comments, but otherwise good to me :). Once that other PR is in so the local media backup section can be updated (if that's good with you) we should be able to merge this.

[tsmethurst]

Cool! This looks good to me, valuable contribution. Feel free to merge once you're both happy with it, i reckon :)

[daenney]

I've pushed some updates, including a little script people can use to transform the output into Borg patterns. This is all pending #1943, otherwise it doesn't make any sense.

[tsmethurst]

Now that #1943 is squerged, should i squerge this too? 👀

[daenney]

Yup. I just rebased it and fixed a few links.