docs: Add examples/README.md and annotate smallest prove/verify demo

## Why
Beginners need a clear prove→verify path and an index of examples.

## Task
1. Add `examples/README.md` listing demos
2. Add brief teaching comments to the smallest prove/verify example

No behavior changes.

## Acceptance
- [ ] Index linked from README
- [ ] Example still runs

## Difficulty
Beginner — docs + comments.

Review the existing demos under examples/ and identify the smallest prove/verify example, then inspect the repository README for the appropriate index link. Add examples/README.md and teaching comments without changing behavior; done means the index is linked from README and the example still runs.

Written by the indexing model from the issue text.