actions/python installs too many requirements

In the "setup python" github action, both dev and docs requirements [are always installed](https://github.com/aai-institute/pyDVL/blob/f4ebb0eaa46b1d7ade249eb19c0fc310f12cbe9a/.github/actions/python/action.yml#L18). But this action is used e.g. in the build docs stage [in the main workflow](https://github.com/aai-institute/pyDVL/blob/f4ebb0eaa46b1d7ade249eb19c0fc310f12cbe9a/.github/workflows/main.yaml#L60), where dev requirements shouldn't be necessary.

Look at the GitHub Actions workflow files: .github/actions/python/action.yml and .github/workflows/main.yaml. The issue is that the 'setup python' action installs both dev and docs requirements unconditionally. In the build docs stage, only docs requirements are needed. Modify the action to conditionally install dev requirements, perhaps by adding an input parameter. Test by running the workflow locally or in a fork to ensure docs build still works and dev dependencies are omitted when not needed.

索引モデルが issue の本文から書いたものです。