doc: clarify where relative devicetree files are found

Add description of how relative devicetree overlay files are picked up
from the application configuration directory.

Also add a link to detailed description of application configuration
directory.

This aligns the devicetree overlays with the description found for
Kconfig configuration files.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>