simplify pre-commit and jupytext settings

[OriolAbril]

Attempt to simplify pre-commit and jupytext config.

This now treats the ipynb as the only source of truth, converting it
to myst when committing. It also removes codespell which ran on myst
files and complicated the whole syncing loop.

• PR description contains a link to the relevant issue: Improve myst workflow #440

[twiecki]

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pymc-examples/envs/483/lib/python3.8/site-packages/sphinx_codeautolink/extension/block.py", line 211, in parse_source
    names = parse_names(modified_source, node)
  File "/home/docs/checkouts/readthedocs.org/user_builds/pymc-examples/envs/483/lib/python3.8/site-packages/sphinx_codeautolink/parse.py", line 21, in parse_names
    tree = ast.parse(source)
  File "/home/docs/.pyenv/versions/3.8.6/lib/python3.8/ast.py", line 47, in parse
    return compile(source, filename, mode, flags,
  File "<unknown>", line 61
    %load_ext watermark
    ^
SyntaxError: invalid syntax

[twiecki]

Had a chance to test it and it works much better 👌.