XWIKI-9176: New XWiki Syntax Guide

 * default to syntax 2.1
XWIKI-9176: New XWiki Syntax Guide

 * XHTML validity: add page name to the sections for
   each syntax version to make the heading's ids unique
 * hide all pages but the main syntax page
XWIKI-9176: New XWiki Syntax Guide

  * CSS coding style cleanup
  * Fix bug where the button for the syntax selector was not hidden
    when a language parameter is present
  * remove dead / commented code