Sphinx apidocs: fix crosslink issues, polish rx.Observable

[erikkemperman]

So finally, after much annoyance and dead ends, I think I have figured out why the cross-links in the sphinx-generated apidocs were not working

• It obviously only works for classes / methods and so on which we document -- so for instance everything in rx.core.typing would never be linked because we weren't refering this module in any rst file.
• There is an issue with re-exporting modules, like is being done for the Observable class -- this lives in rx.Observable, and is referred to as such in e.g. the docstrings in rx/__init__.py, but actually it has a fully qualified name of rx.core.observable.observable.Observable. This confuses the sphinx typehints plugin.

The bad news is that this is a bit hackish (only in docs/conf.py, not in the main code!) but the good news is that if I read this issue correctly some future version will support what I am doing here manually:

https://github.com/agronholm/sphinx-autodoc-typehints/issues/38

I've polished the Observable section a bit, and am planning to do the same for Observable Factory next.

[coveralls]

Coverage increased (+0.001%) to 92.608% when pulling 0d3c3ed on erikkemperman:docs-polish into 862f3cb on ReactiveX:master.

[erikkemperman]

Thanks for review!

I think the Observables page is in reasonably good shape, but if you have some time please take a look -- the more eyes go over it the less likely it'll be that imperfections fall through the cracks. In particular, it would be helpful if you could be on the lookout for parameter and return value types which are not properly linked internally.

(Just rebased to resolve a conflict, the "copyright" I removed was now in a different file. Otherwise no changes).