This commit uses the new .. zephyr:code-sample-category directive to
categorize code samples across the tree.
Updates existing legacy references to manually defined targets to now
use :zephyr:code-sample-category: role instead.
Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
Board samples were listed in a very flat way before.
This commit introduces the appropriate toctrees to fix this.
Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>