doc: fix references in sample apps guide

doc: fix references in sample apps guide

MANY references in the sample applications user guide are wrong because
they are hard-coded and section numbers have changed over the time.
This patch changes thoses references to dynamic ones, in this way if
section numbers change the reference get updated automatically.

Signed-off-by: Mauricio Vasquez B <mauricio.vasquezbernal@studenti.polito.it>