`NotebookValidationError` in CI for deploying docs

[mspronesti]

Building the documentation throws the following error

ERROR:traitlets:Notebook JSON is invalid: Additional properties are not allowed ('execution_count' was unexpected)

Failed validating 'additionalProperties' in markdown_cell:

On instance['cells'][13]:
{'cell_type': 'markdown',
 'execution_count': [37](https://github.com/mspronesti/qlearnkit/actions/runs/3268796549/jobs/5375615382#step:5:38),
 'metadata': {'collapsed': False, 'pycharm': {'name': '#%% md\n'}},
 'source': 'Notice how the quantum boundary is nonlinear. This is a '
           'conseque...'}
ERROR:traitlets:Notebook JSON is invalid: Additional properties are not allowed ('execution_count' was unexpected)

Failed validating 'additionalProperties' in markdown_cell:

On instance['cells'][13]:
{'cell_type': 'markdown',
 'execution_count': 37,
 'metadata': {'collapsed': False, 'pycharm': {'name': '#%% md\n'}},
 'source': 'Notice how the quantum boundary is nonlinear. This is a '
           'conseque...'}
ERROR:traitlets:Notebook is invalid after preprocessor <nbconvert.preprocessors.tagremove.TagRemovePreprocessor object at 0x7f1a29ca87c0>
/home/runner/work/qlearnkit/qlearnkit/qlearnkit/nn/quanv.py:docstring of qlearnkit.nn.quanv.Quanv2DLayer.quanvolve:10: WARNING: Duplicate explicit target name: "quantum long short-term memory".

Notebook error:
NotebookValidationError in tutorials/qsvc.nblink:
Additional properties are not allowed ('execution_count' was unexpected)

Failed validating 'additionalProperties' in markdown_cell:

On instance['cells'][13]:
{'cell_type': 'markdown',
 'execution_count': 37,
 'metadata': {'collapsed': False, 'pycharm': {'name': '#%% md\n'}},
 'source': 'Notice how the quantum boundary is nonlinear. This is a '
           'conseque...'}

This seems to be due to the most recent versions of nbformat and sphinx-gallery.
See this issue for nbformat.

Working versions:

• nbformat 5.4.0
• sphinx-gallery 0.10.1