Bump nbsphinx from 0.8.11 to 0.9.1 in /requirements (#843)

Bumps [nbsphinx](https://github.com/spatialaudio/nbsphinx) from 0.8.11 to 0.9.1.
- [Release notes](https://github.com/spatialaudio/nbsphinx/releases)
- [Changelog](https://github.com/spatialaudio/nbsphinx/blob/master/NEWS.rst)
- [Commits](spatialaudio/nbsphinx@0.8.11...0.9.1)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements/requirements_docs.txt

@@ -2,7 +2,7 @@ ansys_sphinx_theme==0.9.5
 enum-tools[sphinx]==0.9.0.post1
 graphviz==0.20.1
 imageio-ffmpeg==0.4.7
-nbsphinx==0.8.11
+nbsphinx==0.9.1
 pypandoc==1.10
 pytest-sphinx==0.5.0
 pyvista==0.36.1