doc: doxygen: use STRIP_FROM_INC_PATH to keep #includes clean

Another take on zephyrproject-rtos/zephyr#76953.
This uses STRIP_FROM_INC_PATH to make sure #include snippets in the
documentation are not absolute paths and reflect what one would
actually use in their code.

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