Consolidate /doc with /docs

It's confusing to have two docs directories, and cljdocs supports the
/docs path just as well. I don't think this change affects our mkdocs
project.

"When building your docs, cljdoc will look under the doc (else docs)
directory for a cljdoc.edn file in your library’s git
repo."
- https://github.com/cljdoc/cljdoc/blob/master/doc/userguide/for-library-authors.adoc#configuration