Fix Issue #91: Building docs notebooks is now included in the test

• The test stage in the pipeline now includes testing whether the set of notebooks included in the documentation can be built. The job is triggered for every merge request to either the dev or the main branch.

This MR should resolve Issue #91 (closed).

The notebook test is only carried out as a job in the GitLab pipeline. Specifically, the configuration of pytest (the one specified in the tool:pytest section of setup.cfg) remains unchanged, there is no addition to addopts variable. Calling pytest in a local environment by default will not trigger testing the execution of the notebooks in the docs.