DOCS: backup restore improvements (#332)

Mostly add how to speed up restoration by adding activities_visibility_index later. Also some small other improvements.

This is based on what I did on a Pleroma instance. I assume the activities_visibility_index taking so long is still true for Akkoma, but can't really test because I don't have a big enough Akkoma DB yet 🙃

Co-authored-by: ilja <git@ilja.space>
Reviewed-on: https://akkoma.dev/AkkomaGang/akkoma/pulls/332
Reviewed-by: floatingghost <hannah@coffee-and-dreams.uk>
Co-authored-by: ilja <akkoma.dev@ilja.space>
Co-committed-by: ilja <akkoma.dev@ilja.space>