Docs: Add Synthetic Control sensitivity checks walkthrough

[drbenvincent]

Summary

Adds a focused sensitivity-check walkthrough for Synthetic Control to the sc_pymc.ipynb notebook, addressing #789 (follow-up to #749 / #785).

This continues the docs work started in #818 (central guide) and #846 (Staggered DiD walkthrough), keeping the same structure:

• Sensitivity analysis section introducing the pipeline API (EstimateEffect → SensitivityAnalysis → GenerateReport) and linking back to the central guide and pipeline_workflow.
• Related checks for synthetic control subsection enumerating the SC-applicable checks (PlaceboInTime, ConvexHullCheck, LeaveOneOut, PlaceboInSpace, PriorSensitivity) with one-line interpretations and noting which checks belong to other estimators.
• PlaceboInTime deep-dive with how-it-works narrative grounded in citations (abadie2010synthetic, abadie2021using, reichardt2019quasi).
• Walkthrough with numbered steps and pass/fail interpretation guidance.
• "If this check fails" subsection with five concrete diagnostic actions: inspect fold summaries, prune the donor pool, add complementary SC checks, sanity-check the convex-hull condition, probe prior sensitivity.
• Pipeline code cell running PlaceboInTime(n_folds=2, random_seed=seed) on the existing SC dataset together with GenerateReport(include_plots=True).
• HTML report iframe display cell, matching the pattern used in staggered_did_pymc.ipynb.

Notes

• The walkthrough deliberately demonstrates a NOT SUPPORTED placebo-in-time result for the demo dataset (P=0.905, just below the 0.95 default threshold). This ties directly into the "If this check fails" troubleshooting section so readers see the failure path and the next steps.
• Existing cells were left untouched; the new cells were appended after the effect-summary section. The previous trailing empty placeholder cell was removed as part of inserting the new content.

Closes #789

Test plan

• prek run --files docs/source/notebooks/sc_pymc.ipynb passes
• python scripts/validate_notebooks.py docs/source/notebooks/sc_pymc.ipynb passes
• Notebook re-executed end-to-end via jupyter nbconvert --to notebook --execute --inplace ... so all outputs are populated
• CI notebooks job executes the SC notebook without errors
• ReadTheDocs preview renders the new sections correctly

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.59%. Comparing base (deb8774) to head (741434e).

Additional details and impacted files

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- Hits        12076    12074       -2     
- Misses        485      486       +1     
- Partials      203      204       +1     

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Documentation build overview

📚 causalpy | 🛠️ Build #32452428 | 📁 Comparing 741434e against latest (deb8774)

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2 files changed

± 404.html
± notebooks/sc_pymc.html