Improve documentation

This repo is complex and should have more robust documentation, i.e., a [MkDocs](https://www.mkdocs.org/) site for use and development instructions. It should also have contributing guidelines, as alluded to in the README.

Start by reviewing the README and the repository's current documentation structure, then consult MkDocs guidance for a possible site layout. Done means the repository has usable instructions for users and developers, a MkDocs documentation site, and contributing guidelines linked from the README.

Written by the indexing model from the issue text.