fix: pin myst-parser package version to 4.0 to avoid conflicts

[tang-mm]

• Have you updated CHANGELOG.md with relevant non-documentation file changes?
• Have you updated the documentation for this change?

---

myst-parser released 5.0.0 today and caused failures when installing documentation requirements (Python >=3.11):

Example error log:

...
Collecting sphinx-tabs (from -r docs/requirements.txt (line 10))
  Downloading sphinx_tabs-3.4.5-py3-none-any.whl.metadata (6.3 kB)
  Downloading sphinx_tabs-3.4.4-py3-none-any.whl.metadata (6.3 kB)
INFO: pip is looking at multiple versions of sphinx-tabs to determine which version is compatible with other requirements. This could take a while.
...
Collecting Sphinx>=7.1.2 (from canonical-sphinx>=0.5.1->-r docs/requirements.txt (line 2))
  Downloading sphinx-8.2.2-py3-none-any.whl.metadata (7.0 kB)
  Downloading sphinx-8.2.1-py3-none-any.whl.metadata (7.0 kB)
  Downloading sphinx-8.2.0-py3-none-any.whl.metadata (7.0 kB)
  Downloading sphinx-8.1.3-py3-none-any.whl.metadata (6.4 kB)

error: resolution-too-deep
× Dependency resolution exceeded maximum depth
╰─> Pip cannot resolve the current dependencies as the dependency graph is too complex for pip to solve efficiently.

hint: Try adding lower bounds to constrain your dependencies, for example: 'package>=2.0.0' instead of just 'package'.
Link: https://pip.pypa.io/en/stable/topics/dependency-resolution/#handling-resolution-too-deep-errors

[tang-mm]

The workflow "check removed URLs" failed because it checks out two branches: the current branch as "compare" and the merging target branch main as "base". This PR only fixes the build in "compare", but the base branch (main) still has the unpinned version. The failure is expected and can be fixed once this PR is merged.

[AnneCYH]

Confirmed that pinning myst-parser works and the URL check passes after the fix gets merged to the default branch.
Thanks @tang-mm !