Improve the docs

[kwinkunks]

The README is fine but leaves a lot of unanswered questions.

A directory tree diagram showing the structure of a set of courses would be good.

We need to document how the info, hide and exercise cells work in the notebooks. Etc.

[mtb-za]

It looks like you addressed the comments above in 21571c1 ?

We can always add more, but is there anything that you think is obviously outstanding?

[kwinkunks]

This issue was actually not affected by 21571c1

[kwinkunks]

So far we have nothing that details how one should build a notebook for the class, eg using info, hide, and exercise tags.

Should probably do it at least partly by improving the notebooks that are bundled in include (whilst keeping them very small -- but at least one of them should be a more complete demo of the features we use).

And I think a directory tree diagram would be useful, as generated by tree on Linux.