Bump nbsphinx from 0.9.5 to 0.9.6 in /requirements

Bumps [nbsphinx](https://github.com/spatialaudio/nbsphinx) from 0.9.5 to 0.9.6.
- [Release notes](https://github.com/spatialaudio/nbsphinx/releases)
- [Changelog](https://github.com/spatialaudio/nbsphinx/blob/master/NEWS.rst)
- [Commits](spatialaudio/nbsphinx@0.9.5...0.9.6)

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

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

requirements/docs.txt

@@ -1,4 +1,4 @@
 # docs
 ipywidgets==8.1.5
-nbsphinx==0.9.5
+nbsphinx==0.9.6
 sphinx==5.3.0