Bump myst-parser from 3.0.1 to 4.0.0 in /docs (#4434)

Bumps [myst-parser](https://github.com/executablebooks/MyST-Parser) from 3.0.1 to 4.0.0.
- [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...v4.0.0)

docs/requirements.txt

@@ -1,6 +1,6 @@
 # Used by ReadTheDocs; pinned requirements for stability.
 
-myst-parser==3.0.1
+myst-parser==4.0.0
 Sphinx==7.4.7
 # Older versions break Sphinx even though they're declared to be supported.
 docutils==0.20.1