Policy images

Fix for ticket https://trac.openmicroscopy.org.uk/ome/ticket/11215 - screenshots on policies.txt moved to the correct location, renamed, updated and unnecessary ones removed (I didn't see the point in some which were just showing the page that was linked in the text anyway) to continue image tidying and size reduction. Also moved this page to the top of the contributing section as it provides a nice intro to the group.

This file still needs updating re: testing scenarios but I'll do that on a new PR in conjunction with @pwalczysko another time.

N.B. Can be rebased after merging.