Simplify welcome module

Created by: jcfr

As discussed during the last Slicer office hour, this commit simplifies the Welcome module. The associated documentation was out of date and a duplicate of what is already available on the wiki.

After completing the transition to readthedocs (wip: #686), we may revisit and embded the generated html in the application.

The time being, here is the updated UI:

The about and acknowledgement section remain the same.