Add new Squash tutorial and screenshots

As discussed with @sbesson, this tutorial, which was originally under one-internal (see https://github.com/openmicroscopy/ome-internal/pull/79) is now moved to one-documentation, under "contributing". As discussed with @hflynn, develop is the right branch to open it against. This is separate from the tactics on the other scenarios, which will be transferred to Squash software. This is not really a testing scenario, but, as mentioned above, rather a tutorial, and so it is better placed here in the docs. 

/cc @jburel