Remove IE9+ support for docu

docs/Doxyfile

@@ -1056,7 +1056,7 @@ HTML_OUTPUT            = html
 # The default value is: .html.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_FILE_EXTENSION    = .xhtml
+HTML_FILE_EXTENSION    = .html
 
 # The HTML_HEADER tag can be used to specify a user-defined HTML header file for
 # each generated HTML page. If the tag is left blank doxygen will generate a