restructure and improve documentation

* NGINX config is includable and will be installed to
  /etc/nginx/snippets.
* The example NGINX config is basically an include to snippets now.
* Remove sparely maintained production setting examples in
  docs/deployment/settings.py and debian/grouprise.settings.py
  and use the one in grouprise/settings.py.production instead.
* translate all documentation into english and rewrite, complement,
  and extend basically everything
1 job for 650-revise-documentation in 2 minutes and 35 seconds
latest
Status Job ID Name Coverage
  Test
passed #2394
make_test

00:02:35