doc: samples: use :zephyr-app: for in-tree samples

The zephyr-app-commands directive can output a helpful hint to the user
when they are trying to build a sample that is in the Zephyr tree,
telling them to ensure they are in the root folder of the Zephyr repo.

Update all doc pages that were using :app: instead of :zephyr-app: so
that the hint is displayed.

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