[Translations: /de] Initial translation

This is an improved and greatly extended version of the translation started in #1289.

List of things that are not translated yet:
* Entire usage chapter
* Navigation sidebar strings in the Email and Themes chapters (they are hard-coded)

~~I have no clue why Github says it can't automatically merge the branch, it's based on the latest commit. (?)~~
Actually it wasn't, should be fine now.