Add docs/ materials to documentation website (docsite)

[d33bs]

I noticed we have some nice docs within the primary readme and under the docs/ directory. It might be nice for the package audience to consume this information through a documentation website (docsite) such as that which is produced by sphinx or mkdocs. Given that this project is part of a larger family under manubot I wasn't sure if that meant the content should remain isolated here or if it would be better to add it to "organizational" docs for the manubot project.

Benefits here would be that the content might be more easily found and read by people wanting to make use of the project. It also might be beneficial in terms of external SEO (if someone searches using Google, etc.) to help make the docs more easily findable when someone searches outside of GitHub.