[CHORE] Unify contributing guidelines

[EndBug]

It would be nice to have a unified CONTRIBUTING.md file to document how people can contribute to the project, and what is the workflow used by the team to review stuff.

It would also be nice to choose only one language and stick with it for the whole repo
I'm currently writing this in English as I don't know what's the standard, as I see stuff in both Italian and English, both in the issues/PR and in the docs

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Created a ClickUp task linked to this issue: CU-86bwr3ump