Expand CI to include a documentation build that tests against multiple unreleased dependencies at once #3204

The continuous integration suite currently builds the documentation against unreleased versions of upstream dependencies, including sphinx, plasmapy_sphinx, nbsphinx, and sphinx-gallery. However, each build is done using the unreleased version of a single dependency at a time.

We should add a documentation build using the development branches of multiple upstream dependencies simultaneously.

The main use case is when we are trying to fix a problem with plasmapy_sphinx due to a breaking change in sphinx that has not yet been included in a release.

Related issues

PlasmaPy/plasmapy_sphinx#23, PlasmaPy/plasmapy_sphinx#24, PlasmaPy/plasmapy_sphinx#25, PlasmaPy/plasmapy_sphinx#26