docs: add Italian translations

[redenfire]

Add README.it.md and Italian versions of all documentation files (docs/*.it.md), following the existing German translation convention. Update language switcher links in the English docs.

[inventory69]

Hey, thanks a lot for the work you put into this :)

I have to decline it though. I only maintain the docs in English and German, and files like FEATURES, UPCOMING and DOCS change with almost every release. I can't keep an Italian version in sync, so it would be outdated after one or two releases, and outdated docs about sync or encryption do more harm than no translation at all. It would also set a precedent for the other app languages, which I can't maintain either.

If you'd like to help with Italian, the app strings on Weblate are the best place, since every new string shows up there automatically:
https://hosted.weblate.org/engage/simple-notes-sync/