Upgrade documentation to MkDocs

The use of Julia's `Documenter.jl` was an early decision that got us up and running quickly, but we aren't tied to it and it doesn't provide any Julia-specific functionality. Using [MkDocs](https://www.mkdocs.org/) provides better visual appeal as well as a larger feature set (increased section depth, better math mode support, for examples).

No files or tests are named. Start by locating the existing Documenter.jl configuration and documentation entry point, then compare the requirements for an MkDocs setup. Done means the documentation builds with MkDocs and replaces the current Documenter.jl setup.

Written by the indexing model from the issue text.