Bump nbsphinx from 0.8.7 to 0.8.8 in /extra_requirements (#871)

Bumps [nbsphinx](https://github.com/spatialaudio/nbsphinx) from 0.8.7 to 0.8.8.
- [Release notes](https://github.com/spatialaudio/nbsphinx/releases)
- [Changelog](https://github.com/spatialaudio/nbsphinx/blob/master/NEWS.rst)
- [Commits](spatialaudio/nbsphinx@0.8.7...0.8.8)

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

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

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

extra_requirements/requirements-docs.txt

@@ -1,7 +1,7 @@
 # File for the requirements of the documentation
 commonmark==0.9.1
 graphviz==0.19.1
-nbsphinx==0.8.7
+nbsphinx==0.8.8
 recommonmark==0.7.1
 sphinx==4.3.2
 sphinx_rtd_theme==1.0.0