API docs

A simple initial version of API docs, in the folder `apidocs`. This PR adds the html of these docs to the repo, so will be visible locally via (changing the path appropriately): ``` file:///Users/reubencohn-gordon/MicroCanonicalHMC/apidocs/sampling/sampler.html ``` We can easily host them globally, but that requires setting up a Github-Pages website, or using Netlify. Since we also have the other documentation, it would be nicest to host it all jointly, so I'll wait until I incorporate that into the main repo.