Disable Markdown support in our Doxyfile

At least the "Plugin Writing Guidelines" and "Plugin HowTo" sections
are displaying undesirable side effects of Markdown support, so disable
it at least for now.

doc/Doxyfile.in

@@ -265,7 +265,7 @@ EXTENSION_MAPPING      =
 # can mix doxygen, HTML, and XML commands with Markdown formatting.
 # Disable only in case of backward compatibilities issues.
 
-MARKDOWN_SUPPORT       = YES
+MARKDOWN_SUPPORT       = NO
 
 # If you use STL classes (i.e. std::string, std::vector, etc.) but do not want
 # to include (a tag file for) the STL sources as input, then you should