doxygen: Improve C++ ref manual

* Had to use absolute paths otherwise `ortools/<dir>` conflict with `build/ortools/<dir>`
* rework Doxygen main page following devsite C++ ref overview.
* improve doxygen c++ cache
  note: doxygen log specify the ideal cache size according to the previous run.

cmake/cpp.cmake

@@ -579,7 +579,18 @@ if(BUILD_CXX_DOC)
   # add a target to generate API documentation with Doxygen
   find_package(Doxygen REQUIRED)
   if(DOXYGEN_FOUND)
-    configure_file(${PROJECT_SOURCE_DIR}/ortools/cpp/Doxyfile.in ${PROJECT_BINARY_DIR}/cpp/Doxyfile @ONLY)
+    configure_file(
+      ${PROJECT_SOURCE_DIR}/ortools/cpp/Doxyfile.in
+      ${PROJECT_BINARY_DIR}/cpp/Doxyfile
+      @ONLY)
+    configure_file(
+      ${PROJECT_SOURCE_DIR}/ortools/cpp/dirs.cpp.dox.in
+      ${PROJECT_BINARY_DIR}/ortools/dirs.cpp.dox
+      @ONLY)
+    configure_file(
+      ${PROJECT_SOURCE_DIR}/ortools/cpp/main.cpp.dox.in
+      ${PROJECT_BINARY_DIR}/ortools/main.cpp.dox
+      @ONLY)
     file(DOWNLOAD
       https://raw.githubusercontent.com/jothepro/doxygen-awesome-css/v2.3.4/doxygen-awesome.css
       ${PROJECT_BINARY_DIR}/cpp/doxygen-awesome.css
@@ -595,6 +606,8 @@ if(BUILD_CXX_DOC)
         ${PROJECT_SOURCE_DIR}/ortools/doxygen/header.html
         ${PROJECT_SOURCE_DIR}/ortools/doxygen/DoxygenLayout.xml
         ${PROJECT_SOURCE_DIR}/ortools/cpp/stylesheet.css
+        ${PROJECT_BINARY_DIR}/ortools/main.cpp.dox
+        ${PROJECT_BINARY_DIR}/ortools/dirs.cpp.dox
       WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
       COMMENT "Generating C++ API documentation with Doxygen"
       VERBATIM)