DOC: Update Introduction/Software Organization.

More complete overview of subsections.

Fix sections levels related to downloading. Remove redundant overview of
subsections.

Do not discourage Git too much.

Fix CDash url.

Avoid "you"'s.

The Wiki Getting Started pages are out-of-date.

Use the automatically updated version variable.

Dart is no longer.

Updates to the Directory Structure.

Suggest ITKApps Superbuild.

Document the location of archived Doxygen pages.

Consolidate "Join Mailing Lists" and "Documentation" subsections.

Some updates to History.

Change-Id: Id5503c8dc92457b5a9199a8ec942dbbeabc0c17f
Merge "DOC: Update Introduction/Software Organization."