Set up docs build

[meshy]

We shouldn't rely on the README as the only source of documentation.

• use mkdocs-material to build docs.
• verify that the docs build using CI.
• allow previewing built docs before merging. A good first pass at this might be to make the CI site build available as an artefact.

I'm not sure where to host it.

We may want to consider using https://github.com/jimporter/mike to make old versions of the docs available.

The contents of the documentation shouldn't be a blocker for getting this set up --- we can fill it in later.