Simplified welcome module

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 The associated documentation was out of date and a duplicate of what is already available on the wiki.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: PR#686), we may revisit and embed the generated html in the application.

The time being, here is the updated UI implemented in PR#810:



2 posts were split to a new topic: Document modules using mkdocs

Integrated in r26437 to r26441