doc: Add 0MQ version to XHTML11 backend footer

Thanks to Matt Weinstein for the suggestion.

doc/asciidoc.conf

@@ -36,6 +36,20 @@ template::[header-declarations]
 endif::backend-docbook[]
 endif::doctype-manpage[]
 
+ifdef::backend-xhtml11[]
+[footer]
+</div>
+{disable-javascript%<div id="footnotes"><hr /></div>}
+<div id="footer">
+<div id="footer-text">
+ØMQ {zmq_version}<br />
+Last updated {docdate} {doctime}
+</div>
+</div>
+</body>
+</html>
+endif::backend-xhtml11[]
+
 [replacements]
 ifdef::backend-xhtml11[]
 0MQ=ØMQ