Migrate lint/format to Ruff, add CONTRIBUTING.md

[Giuzzilla]

I'm opening this PR to apply lint+formatting on the repo using Ruff and ensure that they get applied consistently over time.

List of changes:

• Migrated lint/format config away from black + isort + pylint to ruff in pyproject.toml
  NOTE: For this I chose some initial sane defaults of Ruff (Pylint errors/warnings + pyflakes + pycodestyle + isort), there are many more rules in Ruff which this repo could adopt incrementally.
• Added dev-requirements.txt to specify dev dependencies like pytest, ruff, etc.
• Actually applied the lint / formatting. This is the change that results in the most changes (even if you applied black / isort / pylint you would have had several changes).
• Added a CI step to verify if linter/formatter have been applied correctly. NOTE: I haven't added this to the test matrix, just running it on Py 3.11, but it can be moved within the matrix.
• Added a pre-commit hook that one can install to avoid accidental non-formatted commits.
• Added a CONTRIBUTING.md explaining both the lint/formatting + how to run tests.

Let me know if this is ok or too many changes (we can tune down the rules, but note that it's less pain to do this now than later).

Ciao!