[Translations: /de] updated upgrading

fixed spelling mistakes and issue #1873
[Translations: /de] added /usage translation
[Translations: /de] added /quickstart translation
[Translations: /de] fixed /usage/faq
[Translations: /de] corrected mostly typos
[Translations: /de] SSH-docs & theme-docs update

added 0.4-theme-related-stuff and the SSH-documentation
[Translations: /de] usage menu and corrections

Finally translated the usage menu section and corrected a mistake I made
a few commits back
Merge pull request #1935 from JoHannesBeere/gh-pages

[Translations: /de] +/usage, +/quickstart, other improvements