doc: sysbuild documentation guide

This is this initial sysbuild guide on what sysbuild is and how to build
a sample using sysbuild.

It provides an architectural overview of sysbuild.
Descriptions on how to use `west build` or `cmake` + `ninja` to build
projects with the sysbuild infrastructure.

Flashing is described through the use of `west flash`.

Extending sysbuild with additional Zephyr based applications are
described, and reference to CMake documentation for including non-Zephyr
based applications are provided.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>