docs(evaluate): Evaluation section index and page structure

[sephmard]

Part of #1436 — Evaluate section restructure.

The evaluation tab already exists but needs a clear entry point that orients readers to the full evaluation workflow before they dive into individual topics (models, harness, benchmarks, etc.).

Tasks

• Revise fern/versions/latest/pages/evaluation/index.mdx to serve as a proper section overview:
  • Explain the Gym evaluation loop end-to-end (choose env → configure model → configure harness → run → collect metrics → analyze)
  • Add navigation cards for each sub-page in the section (models, harness, benchmarks, skills, ablations, aggregate metrics)
  • Link to related sections (Configure Models, Configure Agents, Benchmark Analysis)
• Confirm final page order (position frontmatter) across all pages in the section once sub-pages exist
• fern check passes