doc: integrate about_zephyr -> index to remove warnings

Warnings about duplicate label (_about_zephyr) crept up because of the
way the about_zephyr document was included in the index. This is a
short document, makes sense to have it as part of the index.rst; it
removes clutter from the directory as well as the warning.

Change-Id: Ifdc3a93fb4316be4aef685257d14749769443b41
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>