Add QSE tutorial for the [[5,1,3]] code (#2285)

[sankalpsthakur]

Summary

• Adds an Examples gallery tutorial that uses mitiq.qse on the [[5,1,3]] code, following the core numerical demo from McClean et al. (arXiv:1903.05786 / {cite}McClean_2020_NatComm).
• Compares ideal, unmitigated, and QSE-mitigated $\langle\overline{Z}\rangle$ under single-qubit depolarizing noise, including a small noise sweep plot.
• Wires the tutorial into the Examples gallery and the QSE landing page (matching other technique docs).

This is a focused first tutorial (item 1 of #2285). Stacking QSE with ZNE (item 2) is noted but left for a follow-up, consistent with earlier maintainer guidance.

Fixes #2285

Test plan

• Manually executed the tutorial computation path (~60s); QSE error ≪ unmitigated across $p \in {0.01, 0.02, 0.04, 0.06}$
• DOCS_LITE=1 make -C docs html succeeds; tutorial appears in Examples gallery and QSE guide links
• CI / Read the Docs build executes the new MyST notebook

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