Update Chapter Agenda in German documentation. Rename various doc files.
Update German documentation Einführung, ErsteSchritte, Tutorial and some others. Screenshots are still missing. Add placeholer icons.
Update German documentation - Agenda
Rename files in German documentation, move images, some small changes.
Update German documentation: Motion. Insert small icons. Use linebreaks. Some small updates of other doc files.
Fix #696
Fixed typos. Changed doc style of html and latex output.

- HTML: Changed to twittter bootstrap theme.
- PDF: Improved latex pdf.
- Added doc authors to Impressum section.
- Added required sphinx packages to requirements.txt
Some corrections of the German documentation
Merge pull request #697 from normanjaeckel/Fix_Post_DB_Setup_Signal

Fix #696
Merge pull request #693 from anjahitziger/Handbuch

German documentation