doc: Relocate and revise "System Fundamentals" material

Relocates material that describes the most fundamental terms used
in Zephyr documentation so that it appears in the "Introducing Zephyr"
document. (i.e. Ensures that the terms are defined before they are
used in the "Getting Started Guide" that follows.)

Transforms the material into a definition list to make it easier
for readers to locate and understand the mateial. Revises the text
to give it a consistent look-and-feel.

Change-Id: I0187d99b1bdac37397a4c907d57bc1f24d7698e7
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>