Deploy to readthedocs.

In the reusable workflow deploy_docs.yml, we deploy docs to the GitHub space for documentation hosting, crickets-and-comb.github.io/<repo> (using the gh-pages branch).

Add to deploy_docs.yml an optional, parametrized job that deploys to readthedocs.com. This obviously depends on an account being set up on readthedocs for the consuming repo, hence the need for this job to be optional and to take whatever parameters are necessary to deploy to the right space.

This may also include updating the build-doc make target, or building a new make target, to satisfy readthedocs.