issue #6791 TOC not generated when using a particular Markdown header…

… style

The default for the `TOC_INCLUDE_HEADERINGS` was set to 0, for compatibility this should be 5.
(Problem was observed in the doxygen documentation, chapter markdown, where the TOC was missing; alternatively in the Doxyfile the `TOC_INCLUDE_HEADERINGS` could be set to `5`,. The solution chosen is more consistent with the old situation).

src/config.xml

@@ -667,7 +667,7 @@ Go to the <a href="commands.html">next</a> section or return to the
 ]]>
       </docs>
     </option>
-    <option type='int' id='TOC_INCLUDE_HEADINGS' minval='0' maxval='99' defval='0' depends='MARKDOWN_SUPPORT'>
+    <option type='int' id='TOC_INCLUDE_HEADINGS' minval='0' maxval='99' defval='5' depends='MARKDOWN_SUPPORT'>
       <docs>
 <![CDATA[
  When the \c TOC_INCLUDE_HEADINGS tag is set to a non-zero value, all headings