Utilities/Sphinx: support cmakedomain running in parallel

Fixes: #24076

Signed-off-by: Henry Schreiner <[email protected]>

Utilities/Sphinx/cmake.py

@@ -475,3 +475,4 @@ def setup(app):
     app.add_transform(CMakeTransform)
     app.add_transform(CMakeXRefTransform)
     app.add_domain(CMakeDomain)
+    return {"parallel_read_safe": True}