Docs: add Synthetic Control sensitivity walkthrough

Adds a sensitivity-analysis section to sc_pymc.ipynb that walks through
the pipeline API (EstimateEffect -> SensitivityAnalysis -> GenerateReport)
with PlaceboInTime as the SC default check, and documents the other
SC-applicable checks (ConvexHullCheck, LeaveOneOut, PlaceboInSpace,
PriorSensitivity) with interpretation guidance. Mirrors the structure
introduced for Staggered DiD in #846 and links back to the central guide
from #818.

Includes a runnable pipeline cell and HTML report iframe so the walkthrough
demonstrates an end-to-end placebo-in-time check with concrete pass/fail
narrative and a five-step "if this check fails" troubleshooting block.

Closes #789

Made-with: Cursor

Author: drbenvincent