Build documentation and host via gh-pages

[randomPoison]

Currently the only way to learn the full API for the package is to read through the source code. Ideally, we should be able to generate an online reference for the API and host it publicly via GitHub Pages. As far as I can tell tell there's no "official" documentation generation tool for .NET, so the best options I've found so far are:

• DocFX, which seems to be a relatively new tool for the .NET ecosystem. It's developed publicly as part of the dotnet org on GitHub, so it should probably be the first thing we try.
• Doxygen, old reliable. It's not pretty, it's not especially nice to use, but if we can't get DocFX to work with Unity for whatever reason we'll almost certainly be able to fall back to using Doxygen.

There's already an action for DocFX and one for Doxygen, plus one for updating GitHub Pages sites, so between all of the available tools we should be able to set something up.