[Tails-l10n] (review)[de] index-pages for categories

Hi,
I translated the index pages of the categories of the doc. This branch
only contains the subcategories where all or most pages are already
translated, I missed the last two ones until we have translated enough
there. So please make sure to review all the files contained in my
translation ;)
Please review:

https://gitlab.com/spriver/tails.git
branch: de_index
latest commit: f38f1083 Also added translation

Cheers!
spriver