feat(docs): Add MkDocs template site

[dfinster]

Add a skeleton MkDocs documentation template site.

To run locally:

$ pip install mkdocs
$ pip install mkdocs-material
$ pip install mkdocs-mermaid2-plugin
$ mkdocs serve

[Eta0]

All of our docstring documentation is written as reStructuredText, so mkdocstrings is not currently applicable.

[dfinster]

Closing, no longer needed.