Pull in a copy of gorefs.yaml from go-site and build pages based on it

[pkalita-lbl]

To be done after geneontology/go-site#2233

• Add a mechanism to pull a copy of the latest gorefs.yaml from the master branch of the go-site repository. The file should be placed in the _data directory of this repository. This file should be added to .gitignore because it is just a transient copy of the real source of truth in the go-site repository. The most straightforward way of creating this copy is probably adding a Makefile to this repository with a target that curls a copy from GitHub.
• Use the _data/gorefs.yaml file to generate pages for each GO_REF. If desired we could also add an index page for all GO_REFs. This Jekyll plugin can be used to generate individual pages from data entries: https://github.com/avillafiorita/jekyll-datapage_gen