Add a warning banner to the top of old/obsolete documentation.

One thing I have wanted on the docs for a long while is a big red warning banner, something like:
"this documentation reflects an outdated version of Qt, <link>please click here
for the newest version of Qt and this documentation</link>"

This is useful for when people find documentation from Google or other old links to point them to the 'right' place.

(filed under 'doc', but I guess this may appply to 'developer' with the docs moving there, so tagged it too)