Documentation screenshot workflow scaffold

Motivation

The public MkDocs site needs repeatable screenshot capture before user-facing guides are populated with images. Future documentation work should not depend on ad-hoc page choices or unsafe screenshots that expose tenant-specific values.

User-Facing Change

• Added a User Guide section with page stubs for Dashboard, New Search, Jobs, Results, Browser Terminal, and API Reference.
• Added a Contributor Guide page that defines screenshot capture preconditions, redaction rules, capture steps, and acceptance checks.
• Added a JSON capture manifest that records the initial screenshot target list, viewports, routes, wait selectors, output paths, and guide destinations.

API/IaC Diff Summary

• No API or infrastructure changes.
• MkDocs navigation now includes User Guide and Contributor Guide entries.

Validation Evidence

• uv run mkdocs build passed.