Update nbsphinx requirement from <0.8.7,>=0.8.6 to >=0.8.6,<0.9.4

Updates the requirements on [nbsphinx](https://github.com/spatialaudio/nbsphinx) to permit the latest version.
- [Release notes](https://github.com/spatialaudio/nbsphinx/releases)
- [Changelog](https://github.com/spatialaudio/nbsphinx/blob/master/NEWS.rst)
- [Commits](spatialaudio/nbsphinx@0.8.6...0.9.3)

---
updated-dependencies:
- dependency-name: nbsphinx
  dependency-type: direct:production
...

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

docs/notebook-requirements.txt

@@ -3,7 +3,7 @@ ipython>=8.9.0<8.11.0
 jupyter
 
 # Notebook conversion
-nbsphinx>=0.8.6,<0.8.7
+nbsphinx>=0.8.6,<0.9.4
 # Jinja2 version 3.1 introduced a bug in nbconvert, see https://github.com/jupyter/nbconvert/pull/1737
 Jinja2<3.1