The ODK needs its own reference documentation

Right now, the documentation available on the [ODK’s own website](https://incatools.github.io/ontology-development-kit/) is sparse. Most of the real user-facing documentation is delegated to the OBOOK.

I don’t think delegating all the documentation to the OBOOK is a good idea. At least one obvious problem with that is that it’s very easy for the documentation to fall out of sync with the code.

It may be fine to have tutorials, how-tos, and maybe FAQS on the OBOOK, but _reference documentation_ at the very least should be maintained directly from within the ODK project, independently of the OBOOK.