Pin nbsphinx (#2114)

Fixes: `/home/runner/work/AMICI/AMICI/documentation/example_errors.ipynb:2262: WARNING: undefined label: /example_errors.ipynb#unsuccessful_factorization`

Link was working until recently. Not sure what changed.

Author: dweindl

documentation/rtd_requirements.txt

@@ -4,7 +4,7 @@ mock>=5.0.2
 setuptools==67.7.2
 pysb>=1.11.0
 matplotlib==3.7.1
-nbsphinx>=0.9.1
+nbsphinx==0.9.1
 nbformat==5.8.0
 recommonmark>=0.7.1
 sphinx_rtd_theme>=1.2.0