Derive subsets page automatically from metadata. It is currently static.

Metadata:
https://github.com/geneontology/go-ontology/tree/master/src/ontology/subsets

Of course, this is in a different repo. go.gh.io should have a Makefile that syncs upstream metadata into the _config.yml. See the obo site for examples

Would avoid issues like: geneontology/helpdesk#167

The idea would be to have https://geneontology.org/docs/go-subset-guide/ generated by metadata from the ontology or, more likely, the release cycle (to keep them in sync). A little bit like what we have for the gorefs.

Restarting from old #102.

Tagging @pgaudet and @suzialeksander