Document the goals and the scope of the proof-of-concept

The goals and the scope of the EU OS should be documented on the website:

https://gitlab.com/eu-os/eu-os.gitlab.io/-/blob/main/goals.md?ref_type=heads

Once the list is more or less stable, each goal should be mapped either to a (new) issue (if a specific task needs to be carried out) or possibly to a (new) milestone to track the progress.