Add backup and migration docs

[nicksellen]

I added some backup + migration docs, maybe @dwaxweiler is up to review them?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.72%. Comparing base (08896a6) to head (b0996e3). Report is 6 commits behind head on master.

:exclamation: Current head b0996e3 differs from pull request most recent head 4a9afb3. Consider uploading reports for the commit 4a9afb3 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2704 +/- ## ======================================= Coverage 73.72% 73.72% ======================================= Files 406 406 Lines 44309 44309 Branches 2448 2455 +7 ======================================= Hits 32665 32665 Misses 11644 11644 ```

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[dwaxweiler]

Thanks for the changes, @nicksellen! It's looking good to me now! :+1:

[nicksellen]

Great, I'll merge. But! I actually just remade the docs in their own repo.

So https://docs.karrot.world/ is now deployed from https://codeberg.org/karrot/karrot-docs.

I included this backup and migration content. But from now on we can add stuff to the new repo, contributions welcome!