[FEATURE] update build guide with latest info

[mgovers]

the current build guide in on the CMake project suggests that we use Ninja as our build backend, as well as that we always build everything. This is not up entirely to date anymore:

• we do build using Ninja when doing developer builds (Dev builds) and in CI. In this configuration, we build everything
• we allow users to build the PGM using a clean build environment using a configuration of their choice, since #596 . In this configuration, only the PGM C API is built.

The documentation should be updated.