* Add languages and preprocessing pages
* add content
* address review comments
* make link relative
* update api ref with latest docstrings
* move doc readme and update
* add generator API docs
* fix example code
* design and link fix
Co-authored-by: Malte Pietsch <malte.pietsch@deepset.ai>
Co-authored-by: PiffPaffM <markuspaff.mp@gmail.com>
