Documentation generation

[agoose77]

Documentation should be generated directly from the notebooks using an HTML TemplateExporter. The input and output prompts should be also hidden. Users might want to customise the template, which can already be done by setting c.TemplateExporter.XXX options on the command-line, but we might want to think more carefully about this.

Additionally, we might want to use the export tag to indicate what is code vs what is usage, so that we can hide the source and link to it directly. See nbdev for an example of this. Personally, I think source should be the Jupyter Notebook, but if users feel strongly it might be worth configuring this so that it optionally goes to the generated source. I imagine this would take the form of a base_url option and an extension.