Add a metadata file to each example

[pfebrer]

We could add an extra file with metadata to each example, e.g.:

metadata.yaml:

authors:
    - {name: Some author, email: themail@epfl.ch}
tags:
    - topic:ml_models
    - topic:universal_ml_models
    - software:lammps

with the main goal of not having to manually maintain the .sec files, but also because then we could add a "recipe finder" in the landing page where you could easily filter by the desired tags.