pip-prod(deps): update myst-parser requirement from ~=5.0 to ~=4.0

Updates the requirements on [myst-parser](https://github.com/executablebooks/MyST-Parser) to permit the latest version.
- [Release notes](https://github.com/executablebooks/MyST-Parser/releases)
- [Changelog](https://github.com/executablebooks/MyST-Parser/blob/master/CHANGELOG.md)
- [Commits](executablebooks/MyST-Parser@v5.0.0...v4.0.1)

---
updated-dependencies:
- dependency-name: myst-parser
  dependency-version: 4.0.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/requirements-doc.txt

@@ -1,3 +1,3 @@
-myst-parser~=5.0
+myst-parser~=4.0
 Sphinx~=9.0
 sphinx-rtd-theme~=3.1