Bump myst-nb from 1.3.0 to 1.4.0

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:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]