(for the time being) final fixes to the German translation

[jpue]

Fixes many small things I oversaw when working on #73 and #85, including:

• correction of some more spelling and grammar mistakes
• word changes and small rephrasing to make the terminology consistent within QLC+ and its documentation (as well as their translations)
• typographic adjustments (especially " " to „ “)
• corrected internal links and formal issues with the Markdown syntax
• fixes to many tables whose layout was incorrect due to translation

With that, I think that the (German translation of the) documentation is at least in a usable state, and that further (big) corrections will only become apparent once #81 has been implemented.

[yestalgia]

This would have taken a lot of time, I really appreciate your effort. Thank you.