Clean API docs

[brandenchan]

Current pain point:
No easy way to see changes in local env

• ElasticSearchDocumentStore.init( ) docstring renders incorrectly
• Better API docs rendering format @PiffPaffM
• TransformersReader docstring renders incorrectly
• Better API documentation for PreProcessors
• ESRetriever.retrieve( ) (and other Sister classes)
• Generate API docs once more and push to master
• Add second level navigation to methods - diffrentiate class from methods in right navigation bar @PiffPaffM

optional

• Link to classes and code

[PiffPaffM]

@brandenchan Here are my changes for the docstrings

1. Third level for the docstrings
2. Add Module as key word
3. add Objects as keyword

We could replace Objects by class with some more effort, but it is not possible by just configuring the docstring generation.

Moreover, it is not out of the box possible to use the docstrings of a parent for a child.