cmake: doc: created a doc.cmake CMake module in Zephyr CMake modules dir

Creating a doc.cmake to the new Zephyr CMake modules dir.

This removes the need for `set(NO_BOILERPLATE TRUE)` before loading the
Zephyr CMake package.

It also removes the need within the doc/CMakeLists.txt file to manually
include individual parts of the Zephyr CMake files as this is now
controlled through a single Zephyr CMake doc module.

This aligns the way a Zephyr package is sourced with other places.

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