Add scaladoc to mdoc build

[CloudNiner]

See instructions at https://scalameta.org/mdoc/docs/docusaurus.html#include-scaladoc-in-site

There may be some dragons here, as the docs at that link use docusaurus v1 (not v2) and we're not using the docusaurus sbt plugin.

[CloudNiner]

In the Grand Unified Theory of Documentation this will serve as our "references"