Docs: Add Staggered DiD sensitivity checks walkthrough (follow-up #749)

[drbenvincent]

Follow-up to #749.

Objective

Add focused sensitivity-check documentation for Staggered DiD.

Scope

• Add/update docs section for pre-treatment placebo and related checks
• Document typical failure patterns and recommended follow-ups

Acceptance criteria

• Staggered DiD docs include a clear sensitivity walkthrough
• Includes interpretation and troubleshooting guidance
• Links back to central guide

Parent issue: #785

[arthurmello]

@drbenvincent is anyone working on this already? If not, I'd be happy to take it on :)

[drbenvincent]

Excellent! I'll assign you.

[arthurmello]

@drbenvincent
I'm assuming that the rest of #785 sub-issues will be handled by updating existing notebooks, instead of adding new ones.

For consistency, I'm thinking of adding this to the staggered_did_pymc notebook.

Also, one of acceptance criteria is "Links back to central guide", which relates to #786 (not done yet), so I assume this can be ignored, or that we'll wait for #786 before merging this PR.

Does this make sense?

[arthurmello]

@drbenvincent I've created a PR with a first version of this: #806 . let me know if that's what you had in mind

[drbenvincent]

Hi. Currently ill. Hope to get through the backlog by end of week / start of next week