doc: document the sysbuild PRE/POST_CMAKE, PRE/POST_DOMAINS hooks

Document how sysbuild modules can provide functions that will be invoked
during sysbuild CMake configure time.

The following functions has been documented:
- <module-name>_pre_cmake
- <module-name>_post_cmake
- <module-name>_pre_domains
- <module-name>_post_domains

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