Set up initial documentation

[alexdewar]

At some point, we will probably want the code and notebooks in this repo to have documentation on GitHub Pages.

We will need to work out a few things before we do that, namely:

1. What do we want this repo to be for? Are we going to publish a Python package or is it just example Jupyter notebooks etc.?
2. How are we going to divide the documentation between this repo and the main one? Where should tutorials live?
3. What tooling do we want to use for this? MkDocs is the obvious choice (there is also a plugin for notebooks), but we could also go with mdBook, as we've done for the main repo, for consistency.

My feeling about 2 is that it probably makes sense to have tutorials in this repository, as we will want to walk users through analysing the data, which will be easier if we embed Jupyter notebooks.