On the modular curve \(X_{\text{ndép}}(11)\) On the embedding of \(\text{Gal}(\overline\mathbb{Q}/\mathbb{Q})\) into \(\widehat{GT}\). (Appendix by Michel Emsalem and Pierre Lochak: The action of the absolute Galois group on the moduli space of spheres with four marked points) 