chore: build and deploy Sphinx docs via GitHub Actions

[ArthurBernard]

Summary

• Adds .github/workflows/docs.yml to replace Read the Docs (which wasn't rebuilding)
• On every PR: sphinx-build -W --keep-going validates the doc (CI rouge si warning Sphinx)
• On every push to master: build + deploy automatique sur GitHub Pages

Test plan

• Vérifier que le job build passe sur cette PR
• Merger → vérifier que le job deploy se déclenche et que la doc est accessible sur GitHub Pages
• Désactiver les builds automatiques sur readthedocs.org une fois GitHub Pages validé

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