DOC: Main page was empty

The problem is a result of filtering and as the dox files don't contain "C++" code but just comments, there is no need to run them through the "grouping filter".

Utilities/Doxygen/DoxygenConfig.cmake

@@ -52,7 +52,9 @@ set(DOXYGEN_EXAMPLE_PATH "${DOXYGEN_TEST_DIRS}")
 set(DOXYGEN_EXAMPLE_PATTERNS "*.cxx")
 set(DOXYGEN_EXAMPLE_RECURSIVE "YES")
 set(DOXYGEN_IMAGE_PATH "${ITK_SOURCE_DIR}/Documentation/Art" "${ITK_BINARY_DIR}/Utilities/Doxygen/Modules")
-set(DOXYGEN_INPUT_FILTER "${ITK_DOXYGEN_INPUT_FILTER}")
+set(DOXYGEN_FILTER_PATTERNS
+    "*.h=${ITK_DOXYGEN_INPUT_FILTER}"
+    "*.cxx=${ITK_DOXYGEN_INPUT_FILTER}")
 set(DOXYGEN_FILTER_SOURCE_FILES "YES")
 set(DOXYGEN_SOURCE_BROWSER "YES")
 set(DOXYGEN_REFERENCED_BY_RELATION "YES")