Docs: Fix cell option_spec inheritance

Doesn't actually resolve the `:noindex:` failure, which is how I discovered the option wasn't being inherited, but it is a step in the right direction.

docs/util/cell_documenter.py

@@ -41,12 +41,13 @@ class YosysCellGroupDocumenter(Documenter):
     object: tuple[str, list[str]]
     lib_key = 'groups'
 
-    option_spec = {
+    option_spec = Documenter.option_spec.copy()
+    option_spec.update({
         'caption': autodoc.annotation_option,
         'members': autodoc.members_option,
         'source': autodoc.bool_option,
         'linenos': autodoc.bool_option,
-    }
+    })
 
     __cell_lib: dict[str, list[str] | dict[str]] | None = None
     @property

docs/util/custom_directives.py

@@ -245,12 +245,13 @@ class CellNode(TocNode):
     """A custom node that describes an internal cell."""
 
     name = 'cell'
-
-    option_spec = {
+    
+    option_spec = TocNode.option_spec.copy()
+    option_spec.update({
         'title': directives.unchanged,
         'ports': directives.unchanged,
         'properties': directives.unchanged,
-    }
+    })
 
     doc_field_types = [
         CellGroupedField('props', label='Properties', rolename='prop',