docs: Add a source button for the API documentation to go to the source code on GitHub

[sylvaincom]

Which part of the documentation needs improvement?

API documentation

Describe the problem found in the documentation

Contrary to skore v0.5.1, scikit-learn has this source button from the API documentation that enables to jump to the source code on GitHub (on the video my laptop is lagging a little) :

Enregistrement.de.l.ecran.2025-01-09.a.16.37.04.mov

Suggested improvement

Implement the same Sphinx trick as in scikit-learn.

Additional context

No response

[glemaitre]

The related documentation to turn-on the feature is available here: https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/source-buttons.html

We need to define a couple of variables in the conf.py and it should be fine.