docs: add highlighted walkthrough for instruct-validate-repair examples

[planetf1]

IVR (instruct-validate-repair) is the core Mellea pattern, but it doesn't have a highlighted walkthrough page in docs/docs/examples/ like the other four examples do (data extraction, mify, RAG, telemetry).

The instruct_validate_repair/ directory already has 7 runnable examples covering the full progression from basic generation through requirements, validation, custom validators, and the multi-turn strategy. These would translate well into a step-by-step walkthrough page following the same format as the existing highlighted examples.

What to do:

• Create docs/docs/examples/instruct-validate-repair.md following the same structure as the existing walkthrough pages (frontmatter, concepts covered, prerequisites, section-by-section code walkthrough, key observations, what to try next)
• Pick one or two of the IVR examples as the primary walkthrough (probably 101_email.py progressing to 101_email_with_requirements.py and 101_email_with_validate.py)
• Add a row to the highlighted examples table at the top of docs/docs/examples/index.md

Why it matters:

The four current highlighted examples show generative slots, mify, RAG, and telemetry — all important but secondary to IVR. A new user reading the examples section should see the fundamental pattern front and centre.

Related issues:

• "Making Agents Reliable" Tutorial (Flagship) #526 — "Making Agents Reliable" flagship tutorial (overlapping IVR content)
• docs: Redesign quickstart to focus on a single example #640 — Redesign quickstart to focus on a single example
• feat: self-repairing requirements #454 — Self-repairing requirements