doc: gs: Clearly state the main dependencies min version

Introduce a table with the minimum version required for the three main
dependencies (CMake, Python and DTC). At the same time remove the CMake
help code from the GSG, since it's just a duplicate of the instructions
that Kitware provides.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Co-authored-by: Ruth Fuchss <ruth.fuchss@nordicsemi.no>