Reorganise documentation

[achilleas-k]

Documentation for NIX is scattered across several locations:

• nixio.readthedocs.io: Contains tutorials and documentation automatically generated from the docs/ directory in the repository.
• The GitHub Wiki contains (mostly) technical and developer documentation.
• http://g-node.github.io/nix/ mostly links to other pages (source repo, nixpy docs).

We also have demo repositories that contain helpful scripts and example use cases.

We should organise these, know what each one is for, link to each of them from the repository README interlink them all so that they can be easily discovered by users.

[achilleas-k]

Should combine the API docs with the tutorial examples and push them both to ReadTheDocs.

[jgrewe]

This has been done