Explicitly disallow Python 3 and Ice 3.5 in stable docs

With the new release of Ice 3.5 we need to make it immediately clear that
Ice 3.5 is not (yet) supported. Similarly, Python 3 is not an option,
whether with or without Django involved.

See forum thread:

 * https://www.openmicroscopy.org/community/viewtopic.php?f=5&t=4788&p=9654#p9654