[Tails-dev] new instructions for manual USB installation for…

Hi translators,

The instructions for manual USB installation for Mac are back!

See https://tails.boum.org/doc/first_steps/manual_usb_installation/mac/

Feel free to translate them. While we were at it we also did some
changes to the instructions for Linux so you could have a look at the
fuzzy string in there as well, and review my commit 3f537b9.