doc: add icons to the home page blocks

Use (free license) fontawesome icon characters to pretty up the Zephyr
home page navigation blocks added earlier.  (The fontawesome font famliy
is already included by the read-the-doc theme so we can take advantage
of that.)

Also amend the section descriptions.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>