Cleanup documentation.
Confusing there have been Readme.md and Readme.rst as well as doc/ and docs.
This commit
- makes Readme.md point to the correct installation instructions
- moves a note from doc/index.rst to Readme.md
- moves missing FAQs from doc/index.rst to docs/forword.rst
- moves translation infos from doc/index.rst into new file docs/translation.rst
- removes now empty doc/index.rst, empty doc/de/index.rst Readme.rst and INSTALL