Add visualization of KGCL hierarchy

[caufieldjh]

This is a D3.js viz adapted from @sierra-moxon 's code for the Biolink Model docs (https://biolink.github.io/biolink-model/categories.html)

Generate with make gen-viz, then open src/docs/categories.html.

[caufieldjh]

Remaining oddities:

• Tree diagram cuts off the root node name. Probably needs to be moved to the right a bit.
• Schemaview doesn't find the imported schemas (like prov.yaml) unless they're in the root of the project directory (i.e., kgcl/) but I'd prefer them to stay in the same directory as kgcl.yaml.

[caufieldjh]

Some duplication of what's going into OAK, perhaps: INCATools/ontology-access-kit#700