Building the documentation on Gitlab fails

Building of documentation fails with the following error message:

      $ make html
Running Sphinx v8.0.2
loading translations [en]... done

Extension error:
Could not import extension nbsphinx_link (exception: No module named 'nbsphinx_link')
make: *** [Makefile:20: html] Error 2
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

Probably nbshpinx_link needs to be added to some requirements.