doc: Create "Introducing Zephyr" document

Relocates the introductory material for Zephyr into a new document,
so that it can be more easily located by users. (Previously this
info couldn't be accessed from the list of documents on the left hand
side of a web page; instead, you had to click on the "Documentation"
tab at the top of the page, which is non-intuitive.)

Change-Id: I1e1f78162a314b958789fcef57fc864336007f3c
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>