Make documentation ready for release

[denisalevi]

@SudeshnaBora started setting up a sphinx documentation to be hosted on www.readthedocs.org, see PR #210

The configuration and scripts are based on the brian2 and brian2genn settings.

This issue should stay open until the following documentations are added:

• [ ] Installation instructions, how to use brian2cuda
• [ ] More specific explanation of brian2cuda specific preferences / optimization strategies / examples (currently it just autogenerates a list of all preferences)
• [ ] Add brian2cuda examples (need to check how to add example images, which is happening for brian2 in brian-team/brian2-doc-ressources).
• [ ] Document algorithms? Maybe reference to paper when written is enough here?
• [ ] Limitations if there are things that brian2cuda doesn't support (hope not).

[denisalevi]

Also see #33