docs(readthedocs): :coffin: grafonnet.readthedocs.io exists but is not maintained

[bcdady]

It appears that grafonnet.readthedocs.io was once set up for a place to publish Grafonnet documentation (presumably before GitHub Pages). It's incomplete and doesn't link to an active repository; presumably it was connected to a since-deleted branch.

Perhaps a maintainer can locate the credentials and decommission the site.