More CI cleanup, add style workflow (#107)

- Changes on the `test.yml` workflow:
  - Using poetry instead of pip. Contrary to what I wrote in #75, it is possible to use poetry (and have the benefits of shorter install times) without the need for having two separate versions of `pyproject.toml` and `poetry.lock`.
  - Reduce the trigger scope to only run when files in these directories are modified:
    - `lerobot/`
    - `tests/`
    - `examples/`
    - `.github/`
- Add `style.yml` workflow for doing a `ruff check` pass on the code
- More cleanup (removed deprecated workflow)

CONTRIBUTING.md

@@ -153,17 +153,6 @@ Follow these steps to start contributing:
    poetry lock --no-update
    ```
 
-   **NOTE:** Currently, to ensure the CI works properly, any new package must also be added in the    CPU-only environment dedicated to the CI. To do this, you should create a separate environment and    add the new package there as well. For example:
-   ```bash
-   # Add the new package to your main poetry env
-   poetry add some-package
-   # Add the same package to the CPU-only env dedicated to CI
-   conda create -y -n lerobot-ci python=3.10
-   conda activate lerobot-ci
-   cd .github/poetry/cpu
-   poetry add some-package
-   ```
-
 5. Develop the features on your branch.
 
    As you work on the features, you should make sure that the test suite