Add RD/RKink sensitivity-checks walkthrough

Follow-up to #785: extend rd_pymc.ipynb with a new "Sensitivity analysis"
section that runs `BandwidthSensitivity` + `McCraryDensityTest` via the
pipeline API, and refresh rkink_pymc.ipynb's existing sensitivity section
to match the style established by staggered_did_pymc.ipynb (related
checks, numbered walkthrough, pass/fail interpretation, troubleshooting).
Both notebooks now link back to `pipeline_workflow` and the central
`sensitivity_checks` guide. Docs only; no code changes.

Closes #790

Made-with: Cursor

Author: drbenvincent