Migrate docs site to material for mkdocs

[thobson88]

Partially done on branch mkdocs, and merged into 276-refactor to capture changes outside the docs/ directory (mostly updated docstrings). To be continued on another feature branch after #286.

TODO:

• Set up MkDocs site and translate from Sphinx version
• Fix CI
• Docs homepage
• Getting Started section
• Reference section
• API section
• Experiments section
• Class diagrams & explanation for geoparser modules
• Instructions for running granular tests