Docs: Improve cross-links and navigation for sensitivity checks docs (follow-up #749)

[drbenvincent]

Follow-up to #749.

Objective

Ensure users can find sensitivity-check docs from primary entry points.

Scope

• Add links/callouts from notebook index, workflow docs, and API docs
• Add lightweight pointers in notebooks that do not need full sensitivity sections

Acceptance criteria

• Main docs entry points link to central sensitivity docs
• Navigation is coherent and avoids duplicate long-form content

Parent issue: #785