Add extra script to the docs created in GitHub Actions workflow

This was done by the cron job running in docs.wxwidgets.org before, so
keep doing it here.

docs/doxygen/custom_footer.html

@@ -19,6 +19,8 @@
 
 <script src="wxwidgets.js" type="text/javascript"></script>
 
+<!--EXTRA FOOTER SCRIPT-->
+
 </div><!-- #page_container -->
 </body>
-</html>
+</html>