doc: document CMAKE_BUILD_PARALLEL_LEVEL and VERBOSE env variables

Sysbuild may include several images and when using ninja as build tools
then build options like `ninja -v` or `ninja -j <n>` is only consumed
by the top-level build folder, which is sysbuild itself.

CMake supports setting number of jobs or verbosity using environment
variables.

Document this feature in the Building with sysbuild section.

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