Reorganize repo

[bsipocz]

To separate the content from the site and from the infrastructure.

Some other projects have some nice logic to follow for directory structure, e.g.

https://github.com/networkx/nx-guides

or

https://github.com/numpy/numpy-tutorials

[tomdonaldson]

See user comment from #76:

A possible improvement for the static renderings of the notebooks would be to differentiate between the blank exercises and the worked out exercises. The blank exercises and the corresponding worked out exercises have the same name so it can be confusing. Changing the titles to differentiate between the different workbooks would be helpful.

[tomdonaldson]

With the numpy example above, they are dealing with some issues. Maybe we should wait a few weeks before following that example.

Maybe there is some low-hanging fruit in moving the notebooks from the top level, separating them at least between exercises and solutions for example. Also renaming the notebooks to be more informative could be helpful.

Another minor issue with the rendering is that there is no clear link to the names of the notebooks.