Introduce Onboarding section on README

[ClaudiaComito]

Our Getting started information is quite scattered across the README and other documents.

Identify gaps, summarize available info into a compact and potentially testable (via GitHub Actions) section of the README.

@bducos will create a new branch and draft PR, afterwards all of our new contributors are very welcome to chip in: @AmirTouilPU @neosunhan @SaiSuraj27 @shahpratham @tewodros18 @Mystic-Slice @JuanPedroGHM

[coquelin77]

furthermore, it would be nice to have a short section about how to launch MPI programs. i.e. this is how to launch a code with multiple processes on SLURM (srun/sbatch) or you can run it locally with mpirun/mpiexec.

something like this may be useful for new people who are unfamiliar with MPI codes

[ClaudiaComito]

This is done, though needs constant updates. See #1186