Add ipykernel as an explicit doc dependency

Previously was a transitive dependency from ipywidgets, but that was
removed in favor of a lower level comm package created/used by the
jupyter ecosystem (including ipykernel).

Upon them releasing that change, Circle doc builds began failing as the
nbagg/webagg backends require ipykernel and it was not installed.

Reference https://github.com/jupyter-widgets/ipywidgets/pull/3533/files#diff-d3df85288b79c378bd5be0ae897814abd0db4f90bed160b7259d805a7e77425e

requirements/doc/doc-requirements.txt

@@ -11,6 +11,7 @@ sphinx>=3.0.0,!=6.1.2
 colorspacious
 ipython
 ipywidgets
+ipykernel
 numpydoc>=1.0
 packaging>=20
 pydata-sphinx-theme~=0.13.1