de: update German docs
Created by: rodolfo912
- This PR brings the German docs up to date with the English ones.
- Additionally, I have changed the wording from the more formal Sie form to the more informal Du form, because:
- I think it fits the English tone and the community better (my opinion here has changed since my first translation of the files).
- It is in accordance with existing German translations of other Monero repos (e.g. monero-gui or monero-site).