Use the CMake folder tool to help IDEs groupt targets.

This is done as seen in http://athile.net/library/blog/?p=288

Hopefully this works in Visual Studio as KDevelop doesn't use this feature.

documentation/CMakeLists.txt

@@ -15,6 +15,7 @@ if(SPHINXBUILD_EXECUTABLE)
 		COMMENT "Building PolyVox manual"
 	)
 	add_dependencies(manual doc)
+	SET_PROPERTY(TARGET manual PROPERTY FOLDER "documentation")
 else()
 	if(NOT SPHINXBUILD_EXECUTABLE)
 		message(STATUS "`sphinx-build` was not found. Try setting SPHINXBUILD_EXECUTABLE to its location.")