Updated German version of OpenPGP/GnuPG Best Practices

Took me a while, but I brought the German version of the OpenPGP/GnuPG Best Practices up-to-date with the English version. While doing that I also changed the section on setting your key preferences (generating a new key is not necessary to do this) and fixed some minor stylistic issues (typos, spacing etc.) in the English original.
Unfortunately, during the long time it took me to do this, other people made changes and the best-practices dir has been renamed gpg-best-prafctices (good move), which made a merge necessary.

I shall endeavour to commit quicker and earlier in future.
Yeah, no.
You can actually just set preferences on a key the way I show below. Generating a new key always means collecting signatures from scratch, which is why I think it should be avoided whenever possible.