Review lib docs docstrings demos

In the process of building the new tvb documentation site, we move everything which is up to date in [tvb-root]/tvb_documentation/demos.

Under [tvb-root]/scientific-library/tvb/simulator we kept the folder docs and demos, because not everything was ready to be moved, but these folders are now deprecated and have some code which is not longer interpretable.

Preferable is to have a single location for the demos, cleaned of everything old and no longer maintained. Opinions of what this location should be are welcome.