doc: Update sample.tmpl with new directive

Update the sample.tmpl file so that it encourages people to use the new
:zephyr:code-sample: directive.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

doc/templates/sample.tmpl

@@ -1,11 +1,13 @@
-.. _descriptive_title_link_name:
+.. zephyr:code-sample:: a_unique_id_for_the_sample
+   :name: A descriptive short name for the sample
+   :relevant-api: space-separated list of Doxygen groups of APIs this sample is a good showcase of
 
-[A Descriptive Title]
-#####################
+   Short text description of the sample. It is recommended to word this as if you were completing
+   the sentence "This code sample shows how to ...").
 
 Overview
 ********
-[A short description about the sample and what it does]
+[A longer description about the sample and what it does]
 
 Requirements
 ************