Update myst-parser requirement from >=3.0.1 to >=4.0.1 in /requirements

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@v3.0.1...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]

requirements/doc.txt

@@ -1,4 +1,4 @@
 -r towncrier.txt
-myst-parser >= 3.0.1
+myst-parser >= 4.0.1
 sphinx==8.2.3
 sphinxcontrib-towncrier