doc: Move doxygen intro to file for USE_MDFILE_AS_MANPAGE

doc: Change header to notitle

doc/Doxyfile.in

@@ -790,7 +790,7 @@ WARN_LOGFILE           =
 # spaces. See also FILE_PATTERNS and EXTENSION_MAPPING
 # Note: If this tag is empty the current directory is searched.
 
-INPUT                  = src
+INPUT                  = src doc/README_doxygen.md
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses
@@ -974,7 +974,7 @@ FILTER_SOURCE_PATTERNS =
 # (index.html). This can be useful if you have a project on for instance GitHub
 # and want to reuse the introduction page also for the doxygen output.
 
-USE_MDFILE_AS_MAINPAGE =
+USE_MDFILE_AS_MAINPAGE = doc/README_doxygen.md
 
 #---------------------------------------------------------------------------
 # Configuration options related to source browsing