Bump nbsphinx from 0.8.9 to 0.9.4

Bumps [nbsphinx](https://github.com/spatialaudio/nbsphinx) from 0.8.9 to 0.9.4.
- [Release notes](https://github.com/spatialaudio/nbsphinx/releases)
- [Changelog](https://github.com/spatialaudio/nbsphinx/blob/master/NEWS.rst)
- [Commits](spatialaudio/nbsphinx@0.8.9...0.9.4)

---
updated-dependencies:
- dependency-name: nbsphinx
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

docs/requirements.txt

@@ -2,7 +2,7 @@ sphinx==5.3.0
 furo==2022.9.29
 myst-parser==0.18.1
 autodoc-pydantic==2.2.0
-nbsphinx==0.8.9
+nbsphinx==0.9.4
 sphinx-copybutton==0.5.*
 
 # update when this is resolved: https://github.com/spatialaudio/nbsphinx/issues/655