Add German translation

[mattesmohr]

Jonas (@Suboptimierer) will help me with the translations.

We have decided to merge the current PR #668, as we think, it is better to merge the upcoming translations after another, instead of holding them back, till we finish all of them.

I open the issue for organisation reason and to document the progress:

Index

File	Status	Progress	Editor
index.md	Done	100%	Mattes

Install

File	Status	Progress	Editor
linux.md	In Progress		Mattes
macos.md	In Progress		Mattes

Getting Started

File	Status	Progress	Editor
hello-world.md	Done	100%	Mattes
folder-structure.md	Open
spm.md	Done	100%	Mattes
xcode.md	Done	100%	Mattes

Basics

File	Status	Progress	Editor
routing.md	In Progress	90%	Mattes
content.md	Done	100%	Mattes
client.md	Open
validation.md	Done	100%	Mattes
async.md	In Progress	90%	Mattes
logging.md	In Progress	90%	Mattes
environment.md	Open
errors.md	Open
controllers.md	Done	100%	Mattes

Fluent

File	Status	Progress	Editor
overview.md	Open
model.md	Open
relations.md	Open
migration.md	Open
query.md	Open
transaction.md	Open
schema.md	Open
advanced.md	Open

Leaf

File	Status	Progress	Editor
getting-started.md	In Progress		Jonas
overview.md	In Progress		Jonas
custom-tags.md	In Progress		Jonas

Redis

File	Status	Progress	Editor
overview.md	In Progress		Jonas
sessions.md	In Progress		Jonas

Advanced

File	Status	Progress	Editor
middleware.md	Open
testing.md	Open
server.md	In Progress	90%	Mattes
files.md	Open
commands.md	Open
queues.md	Open
websockets.md	Open
sessions.md	Open
services.md	Open
apns.md	Open

Security

File	Status	Progress	Editor
authentication.md	In Progess	50%	Mattes
crypto.md	In Progess		Jonas
passwords.md	In Progess		Jonas
jwt.md	In Progess		Jonas

Deploy

File	Status	Progress	Editor
digital-ocean.md	Open
heroku.md	In Progress		Jonas
supervisor.md	Open
systemd.md	Open
ngix.md	Open
docker.md	Open

Contributing

File	Status	Progress	Editor
contributing.md	Done	100%	Jonas

Version (4.0)

File	Status	Progress	Editor
legacy-docs.md	Open
upgrading.md	Open

Release Notes

File	Status	Progress	Editor
release-notes.md	In Progress		Jonas