Revisit content “See 3D Slicer in Action” and “What is 3D Slicer?” sections

[jcfr]

from @mhalle

“See 3D Slicer in Action” has the first pictures of Slicer. They are all somewhat niche applications. I think it would be better to have a collection of pictures of Slicer’s core functionality first as part of the “What is 3D Slicer?” section. Slicer is a graphical program, “what is Slicer?” Is answered with a graphical answer. As such, features (with images) should move up, See 3D Slicer in Action should move down.