[Tails-dev] [review'n'merge] Fix alt text

Hello, in the "Introduction to GNOME and the Tails desktop" page some
alt texts for images are useless and one alt text for a button image is
missing.

Please review and merge.

Repo: https://git-tails.immerda.ch/xin/tails/
Branch: fix_alt_text
Last Commit: 11754dfafbfc2506ef87b6a0a5a4ca2da807136a

Cheers.
xin