CONTRIBUTING.md could be simplified a little

can we make an executable that does all of this

```bash
# black format only the source code
black fastplotlib/

# run tests from the repo root dir
RENDERCANVAS_FORCE_OFFSCREEN=1 pytest tests/

# desktop examples
pytest -v examples

# notebook examples
FASTPLOTLIB_NB_TESTS=1 pytest --nbmake examples/notebooks/
```
