Add RD/RKink sensitivity-checks walkthrough

[drbenvincent]

Closes #790

Summary

Follow-up to #785: documents how to use the pipeline-style sensitivity checks for {term}`Regression discontinuity design` and {term}`Regression kink design` workflows, mirroring the staggered-DiD walkthrough that landed in `staggered_did_pymc.ipynb`.

• New Sensitivity analysis section in `rd_pymc.ipynb` with a Related-checks summary, numbered walkthrough, executed pipeline cell running `BandwidthSensitivity` + `McCraryDensityTest`, an HTML-report iframe, pass/fail interpretation, and an If these checks fail troubleshooting block.
• Refresh of the existing sensitivity section in `rkink_pymc.ipynb` with the same structure: links to `pipeline_workflow` and the central `sensitivity_checks` guide, a Related-checks block clarifying that `McCraryDensityTest`, `PlaceboInTime`, `LeaveOneOut`, and `PlaceboInSpace` are not applicable to RKink, a numbered walkthrough, pass/fail interpretation, and an If this check fails troubleshooting block.
• Docs only — no code changes.

Notes

• The bandwidth sweep in `rd_pymc.ipynb` uses the linear interaction formula `y ~ 1 + x + treated + x:treated` rather than basis splines, with an inline note explaining why: `bs(x, df=6)` bakes its knots into the full data and patsy raises an error if a narrower bandwidth subset asks the model to evaluate observations that fall outside those baked-in outer knots.
• The McCrary check fails on the bundled `rd` dataset purely because the running variable spans `[-1, 1]` while the threshold sits at `0.5`. The notebook flags this explicitly so readers don't read the failure as evidence of manipulation in the example data.

Test plan

• `prek run --all-files` passes (notebook schema, ruff format, codespell, mypy, BibTeX, etc.)
• Both notebooks validate with `nbformat.validate`
• `rd_pymc.ipynb` sensitivity pipeline cell executes successfully (real outputs committed)
• CI passes on this PR
• Visual review of the rendered docs page

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✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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