images: Add a new image for the home page

This artwork was made for us by Jacqueline Wong and myself, and we
release it under a Creative Commons Attribution Share-Alike 3.0 Unported
license.

Sources are included, for others to reuse, modify, improve, etc...

We call it "Cangjiception", as it represents "Cangjie" in Cangjie.
index: Use the Cangjiception image

This makes our home page a bit less bare, a bit more welcoming and fun.