Remove the sphinx.ext.githubpages extension (#566)

This PR removes the `sphinx.ext.githubpages` extension from the Sphinx
configuration file. That extension supplies a `.nojekyll` file, that's
useless in practice because of the way we're managing docs in this
repository - we only need a single `.nojekyll` file at the root of the
repository.

docs/source/conf.py

@@ -35,7 +35,6 @@
 extensions = [
     "sphinx.ext.autodoc",
     "sphinx.ext.extlinks",
-    "sphinx.ext.githubpages",
     "sphinx.ext.intersphinx",
     "sphinx.ext.napoleon",
     "sphinx.ext.viewcode",