Improve API Reference Accuracy and Navigation

A cleanup pass on the API reference:

• Response schemas no longer declare fields as required — that marker now appears only on request inputs, where it actually constrains what you send. If you generate clients or validate responses strictly, regenerate against the updated spec.
• The Funnel Stats and Page Stats endpoints are now documented under the Funnel and Page sections, so everything about a resource lives in one place.
• current_path on Page responses is now correctly typed as nullable — externally-hosted pages return null there — and the external-page create example mirrors a real payload.

Resources: Funnel · Page

Building with an AI agent? The Stats skill covers reading funnel and page analytics.