chore(ci): add markdown-link-check action

[jeertmans]

Add a check to prevent dead links in the future (e.g., #937).

Output of markdown-link-check CONTRIBUTING.md README.md > output.txt 2>&1:

FILE: CONTRIBUTING.md
  [✓] https://cla.developers.google.com/
  [✓] https://help.github.com/articles/about-pull-requests/
  [✓] https://opensource.google.com/conduct/

  3 links checked.

FILE: README.md
  [✓] http://optax.readthedocs.io
  [✓] https://optax.readthedocs.io/
  [✓] https://optax.readthedocs.io/en/latest/api/optimizers.html
  [✓] https://optax.readthedocs.io/en/latest/api/losses.html
  [✖] https://github.com/google-deepmind/optax/blob/main/docs/optax-101.ipynb
  [✓] https://arxiv.org/pdf/2306.07179
  [✓] https://proceedings.mlr.press/v139/schmidt21a
  [✓] https://arxiv.org/abs/2206.13424
  [✓] https://github.com/google-research/tuning_playbook
  [✓] https://github.com/google-deepmind/optax/actions/workflows/tests.yml/badge.svg?branch=main
  [✓] https://readthedocs.org/projects/optax/badge/?version=latest
  [✓] https://img.shields.io/pypi/v/optax

  12 links checked.

  ERROR: 1 dead links found!
  [✖] https://github.com/google-deepmind/optax/blob/main/docs/optax-101.ipynb → Status: 404

[vroulet]

Thanks!