doc: supporting module Kconfig code in module ext root

Fixes: #31486

Sourcing `cmake/extensions.cmake` to have Zephyr functions available
when including Zephyr CMake files into doc build system.

This is a follow up on #30904 by supporting generated Kconfig files
that might contain lines as `osource "$(ZEPHYR_<MODULE_NAME>_KCONFIG)"`
to be properly parsed for doc building.

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