Various fixes

Merged Maxim Karasev requested to merge mxkrsv/user-handbook:updates into master

This fixes what I found at first sight.

I plan to do some refactoring after that, as I find that docs are too verbose sometimes (i.e. teaching user how to enter BIOS) and sometimes they suggest controversial things (like advising to do timezone setup manually instead of setup-timezone without a specific reason).

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