add doctest sphinx extension in conf.py so doctests can run

doc/conf.py

@@ -28,7 +28,7 @@
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ["sphinx.ext.autodoc", "sphinx_copybutton"]
+extensions = ["sphinx.ext.autodoc", "sphinx.ext.doctest", "sphinx_copybutton"]
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ["_templates"]