Update nbsphinx requirement from ~=0.9.5 to ~=0.9.6

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@0.9.5...0.9.6)

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

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

requirements/docs.txt

@@ -1,6 +1,6 @@
 sphinx ==5.3.0
 myst-parser ==1.0.0
-nbsphinx ~=0.9.5
+nbsphinx ~=0.9.6
 ipython[all] ~=8.30.0
 pandoc ==2.4
 docutils >=0.16