feat: Add /docs used to deploy documentation to GitHub Pages

[thomass-dev]

Following the official documentation, i propose the current workflow to build the sphinx documentation:

1. (in the current iteration)
   Use a dedicated docs/ directory to store documentation artifacts that will be automatically rendered by GitHub on GitHub Pages.
   Every time something changes in this directory on the main branch, the GitHub Pages will be refreshed.
   So you can manually edit the site content on a PR, and push that PR on main to be applied.
   Documentation can be managed independently of release.

2. (in the next iteration)
   Populate docs/ by triggering sphinx in a GitHub action when something changes in the skore/ directory.

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[thomass-dev]

We should think about how to provide documentation for each version of skore (like the drop-down selection of python documentation) in a final iteration.