Bump nbsphinx from 0.9.3 to 0.9.4 in /docs

Bumps [nbsphinx](https://github.com/spatialaudio/nbsphinx) from 0.9.3 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@0.9.3...0.9.4)

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

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

docs/requirements.txt

@@ -1,5 +1,5 @@
 ipykernel==6.25.2
-nbsphinx==0.9.3
+nbsphinx==0.9.4
 pydata-sphinx-theme==0.14.4
 sphinx==7.1.2
 sphinx-gallery==0.15.0