Doc: Online template: dynamically load footer content

Add a line to load footer content from the global style folder.

Change-Id: If57f13c2301e6145a26b3a4d04c7cb4695fe55fe
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>

doc/global/html-header-online.qdocconf

@@ -36,6 +36,7 @@ HTML.headerscripts = \
     "  <script type=\"text/javascript\" src=\"scripts/extras.js\"></script>\n" \
     "  <script type=\"text/javascript\">\n" \
     "  \$(function(){\n" \
+    "     \$(\"#footer\").load(\"/style/qt-footer.html\");\n" \
     "     \$(\"#sidebar-content\").load(\"style/qt5-sidebar.html\");\n" \
     "  });\n" \
     "  </script>\n" \