Support only directives that impact the TOC

This addresses one issue noted in #132 although due to Sphinx bug
sphinx-doc/sphinx#9819 it is still not
possible to use only directives to selectively include `genindex` and
`modindex` in the global TOC.

sphinx_immaterial/nav_adapt.py

@@ -248,6 +248,10 @@ def visit_reference(self, node: docutils.nodes.reference):
         self._url = node.get("refuri")
         raise docutils.nodes.SkipChildren
 
+    # `only` directives can result in `comment` nodes.
+    def visit_comment(self, node: docutils.nodes.comment):
+        raise docutils.nodes.SkipChildren
+
     def visit_compact_paragraph(self, node: docutils.nodes.Element):
         pass