DOC: Clear warnings when building docs

[david-cortes-intel]

Description

This doc clears some warnings thrown by sphinx when building the documentation for sklearnex.

The following warnings should now disappear:

<path>/scikit-learn-intelex/doc/sources/kaggle/classification.rst:18: WARNING: Duplicate explicit target name: "support vector classification (svc) for multiclass classification".
looking for now-outdated files... none found
pickling environment... done
checking consistency... <path>/scikit-learn-intelex/doc/sources/samples/daal4py_data_science.ipynb: WARNING: document isn't included in any toctree
<path>/scikit-learn-intelex/doc/sources/samples/tsne.ipynb: WARNING: document isn't included in any toctree
done
preparing documents... WARNING: unsupported theme option 'display_version' given

Note: here I'm switching Sphinx option display_version to version_selector, as suggested in their docs:
https://sphinx-rtd-theme.readthedocs.io/en/stable/configuring.html#confval-display_version

If I understand the docs correctly, this display_version option shouldn't actually be doing anything when building with the version of Sphinx in the locked requirements, but they mention that this option requires the ReadTheDocs add-on, which I don't know if is used by the CI job that builds the docs:
https://sphinx-rtd-theme.readthedocs.io/en/stable/configuring.html#confval-version_selector

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Checklist to comply with before moving PR from draft:

PR completeness and readability

• I have reviewed my changes thoroughly before submitting this pull request.
• I have updated the documentation to reflect the changes or created a separate PR with update and provided its number in the description, if necessary.
• Git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details).
• I have added a respective label(s) to PR if I have a permission for that.
• I have resolved any merge conflicts that might occur with the base branch.

Testing

• I have run it locally and tested the changes extensively.
• All CI jobs are green or I have provided justification why they aren't.

Performance

Not applicable

[icfaust]

I'm going to step back from reviewing this one because I don't have a good handle on the docs. Sorry.

[Vika-F]

Thanks for fixing the warnings!
Please take a look at the comments below.

[Vika-F]

Let's get green CI and merge.