Explain APIMart across searchable conversion pages

[luyx-66]

What changed

• Replaced bare APIMart conversion links with clear positioning: a multi-model AI API gateway with an OpenAI-compatible endpoint.
• Added guidance to compare current pricing and choose a cost-efficient model option before the registration CTA.
• Applied the messaging to the app homepage, GitHub Pages homepage, 50 guides, 10 measured comparisons, repository READMEs, and benchmark documentation.
• Added validation and rendered-page tests so future generated content cannot ship with an APIMart link but no platform explanation.
• Refreshed the generated content manifest for 2026-07-22.

Why

Visitors arriving from search should understand what APIMart provides and why the registration link is relevant before being asked to create an account.

Validation

• npm test
• npm run lint
• Coverage audit: 63/63 searchable Markdown documents and 61/61 public HTML pages contain both APIMart positioning and a tracked registration link.