doc/getting_started: replace missing TOC entry with link

A warning was generated for the TOC entry "About Zephyr", which was
missing; replaced with a link to said section (to reduce clutter in
the document).

Change-Id: I1f879bc21bad8ec857781208ad9e31079015292a
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>