Deprecate omero_client.jar.

This PR aims to clear up the confusion around ``omero_client.jar`` (see https://www.openmicroscopy.org/community/viewtopic.php?f=6&t=7573).

To test, make sure that the new guidelines are clear and concise.

@sbesson: Do we want to make ``OMERO.java.zip`` more prominent on the download page?
/cc @joshmoore
change "part" for "section" or somesuch?