build(deps): 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] <[email protected]>

docs/requirements.txt

@@ -2,7 +2,7 @@ appdirs==1.4.4
 furo==2024.4.27
 ipython>=7.0.0
 myst_parser==3.0.1
-nbsphinx==0.9.3
+nbsphinx==0.9.4
 numpy>=1.20.0
 py21cmemu>=1.0.5.dev0
 scipy>=1.7.0