Fixed github language attribute for project by changing CSS file size
* Description
    * Change to minimize the size of the CSS file, so
      that github thinks this is a C++ project instead
      of a CSS project
    * Leverages new doxygen feature HTML_EXTRA_STYLESHEET
* Changes
    1. upgrade doxygen.conf
    2. git mv doxygen.conf dox.conf
    3. git mv doxygen.css dox.css
    4. Modify dox.conf HTML_EXTRA_STYLESHEET
    5. Modify Makefile dox target for dox.conf