update & expand docs for "Direct Backups"

[brontolosone]

Towards issue getodk/central#1646
Documentation part of PR getodk/central-backend#1760

What is included in this PR?

Update & expansion of the "Backing Up Central" page of the docs.

What problems did you encounter?

I've put "advanced" content inside <details> elements so that they're not visible at first sight, lest they overwhelm, but I couldn't quite work out how to make the <summary> part a proper RST section :-/

[brontolosone]

CI build flips out with:

sphinx.errors.ExtensionError: Could not import extension sphinx_toolbox.collapse (exception: cannot import name 'logger' from 'sphinx.ext.autodoc' (/home/circleci/.local/lib/python3.11/site-packages/sphinx/ext/autodoc/__init__.py))

But I'm using the ::collapse I've found elsewhere in the docs, so...

[lognaturel]

The issue you're seeing is an autodoc incompatibility with sphinx 9. Rebasing on master should fix it.

[brontolosone]

This used to be only coincidentally (sometimes) true. Now it's actually true. Restoring to a Central instance will entirely replace all of its data. (Or not, if something goes wrong. But then it doesn't leave you halfway as it's transactional now. The previous restore wasn't transactional).

Furthermore previously we asked "to make sure you're restoring to the right place" but there was not any way to find out what a particular archive's destination DB was. You just had to run the restore and see if you had any new databases in Postgres (or see if some DB had been overwritten ⚡ ), so that was an impossible task.