docs: Redesign quickstart to focus on a single example

[planetf1]

Context

The quickstart page currently has 3 separate examples, which dilutes the "get started fast" experience. Reviewer feedback from PR #601 suggests consolidating to a single, focused example.

Raised in PR #601 review:

• H3/X2-a — comment: What should users take away from the quickstart?
• H3/X2-b — comment: Consolidate 3 examples down to 1
• H3/X2-c — comment: "What's next" section feels out of place — link out instead

Proposed approach

1. Pick one example that best showcases Mellea's value proposition (likely the IVR loop — instruct + requirement + repair)
2. Remove the other examples or move them to tutorials
3. Keep the quickstart to < 1 screen of code + explanation
4. End with clear "next steps" links to tutorials and concepts

Files

• docs/docs/getting-started/quickstart.md (primary)
• docs/docs/index.mdx (may need landing page links updated)
• docs/docs/docs.json (nav if pages move)

Acceptance criteria

• Single focused example in quickstart
• Runs end-to-end with Ollama + granite4:micro
• Clear next-steps links