Python wrapper: documentation (#799)

Add docs for Python wrapper to ReadTheDocs:

- Overview, Common Types, Hardware Management, Recipes, Domain Decomposition, Simulations, Metering
- Installing Arbor: Optional Requirements (Python), Buidling and Installing (Python Frontend), and Installation (Python Module)

Missing (, since not implemented yet): 

- probes
- arbor-sup 
- hint maps in domain_decomposition
- reset, events, empty schedule in event_generator
Also does not cover unit testing (since doc is user-centric).

Makes also defaults and units in wrapper consistent.

Fixes  #766