Development environment setup guide should suggest `requirements-docs.txt`

[zevisert]

Pre-commit currently requires the dependencies from requirements-docs.txt. The contributors guide at https://unionml.readthedocs.io/en/latest/contributing_guide.html#development-environment-setup suggests to install only requirements-dev.txt. Used pyenv to make my venv, but there's nothing special about that.

> pyenv install 3.7.13
[...]
> pyenv virtualenv 3.7.13 unionml 
[...]
> pyenv local unionml
[...]
> pip install -r requirements-dev.txt
[...]
> pre-commit install
[...]
> pre-commit run --all
flake8...................................................................Passed
black....................................................................Passed
isort....................................................................Passed
check yaml...............................................................Passed
fix end of files.........................................................Passed
trim trailing whitespace.................................................Passed
shellcheck...........................................(no files to check)Skipped
mypy.....................................................................Passed
convert myst markdown to jupyter notebooks...............................Failed
- hook id: convert-myst-to-ipynb
- exit code: 1

Traceback (most recent call last):
  File "/opt/tooling/pyenv/versions/3.7.13/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/opt/tooling/pyenv/versions/3.7.13/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/code/oss/github/unionai-oss/unionml/scripts/myst_to_ipynb.py", line 14, in <module>
    import jupytext
ModuleNotFoundError: No module named 'jupytext'