chore: remove references to black from the project

[fpgmaas]

Description

There are a few mentions of black in the project, but they seem inconsistent;

• In .github/workflows/docs.yml, we use black to validate the formatting of Python code in the docs. However, there is no set-up or instructions to format code locally. Of course this is not a big issue; fixing the formatting is trivial when this gives an error by installing black.

  - uses: psf/black@stable
    with:
      src: docs/src/python

• In .github/workflows/python_build.yml black seems to be installed but unused.
• There is some configuration for black in python/pyproject.toml, even though the formatter for the project is ruff.

This PR aims to solve these inconsistencies:

• Remove the references to black listed above.
• Use ruff as a formatter for the docs to be consistent with the python directory.
• Add a small Makefile to the docs directory, with a format and a check command.

Considered alternative

I also considered adding the make commands to python/Makefile, similar to the build-docs command:

.PHONY: format-docs-py
format-docs-py: ## Format python files in the docs directory
    $(info --- format Python code in docs ---)
    (cd ../docs; ruff format .)

.PHONY: check-docs-py
check-docs-py: ## Check if python files in the docs directory are formatted
    $(info --- check Python code in docs ---)
    (cd ../docs; ruff format --check .)

However, this solution has the disadvantage that we need to either install the complete virtual environment in the docs.yml workflow, or hardcode the version of ruff there, e.g.

  lint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - run: |
          cd python
          # Should be the same as in python/pyproject.toml
          pip install ruff==0.5.2 
          make check-docs-py

[fpgmaas]

I believe CI/CD is failing due to this issue. Merging this PR will solve it for now.