DOC: Update Introduction/Organization

Better names for Part II and III that give a more explicit description for
what they contain and decrease the focus on User/Developer divide.

Tcl wrapping is no longer tested.

Part II has system features illustrated with examples, but the vast collection
of cookbook examples are being migrated to other collections.

There are no pre-compiled binaries.

Change-Id: I0d7003b69c08774bd12765ff6a11a4af46e8cb77
DOC: Updates to Introduction/How to learn ITK.

Describe the three categories of documentation - guide, examples and Doxygen.

Suggest three stages - Introduction, examples and documentation overview, and
mastery.

Change-Id: I88d9c312a73e47620b34aa65de0f9e43c465aeb8
Merge "DOC: Update Introduction/Organization"
Merge "DOC: Updates to Introduction/How to learn ITK."