doc: Edit About Zephyr for readability.

Edit the content, adding bold text and headings for readability.
Revised the structure based on comments by developers.
Resubmitted to solve merge issues.

Change-Id: I7679eb7c71eab5cd18addcdb296c376923984494
Signed-off-by: L.S. Cook <leonax.cook@intel.com>
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>