Bump myst-nb from 1.3.0 to 1.4.0 in /docs

Bumps [myst-nb](https://github.com/executablebooks/myst-nb) from 1.3.0 to 1.4.0.
- [Release notes](https://github.com/executablebooks/myst-nb/releases)
- [Changelog](https://github.com/executablebooks/MyST-NB/blob/main/CHANGELOG.md)
- [Commits](executablebooks/MyST-NB@v1.3.0...v1.4.0)

---
updated-dependencies:
- dependency-name: myst-nb
  dependency-version: 1.4.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -3,7 +3,7 @@ autodoc-pydantic==2.2.0
 furo==2025.12.19
 linkify-it-py==2.0.3
 matplotlib==3.10.8
-myst-nb==1.3.0
+myst-nb==1.4.0
 sphinx==8.2.3
 sphinx-click==6.2.0
 sphinx-copybutton==0.5.2