docs: Add circuits/README.md template index with include paths

## Why
Many templates under `circuits/`; newcomers need include paths in one place.

## Task
Add `circuits/README.md` listing major templates (hashing, merkle, comparators, voting, etc.) with example `include "..."` lines.

## Acceptance
- [ ] Paths match tree
- [ ] Linked from main README

## Difficulty
Beginner — documentation only.

Start by inspecting the circuits/ tree and the existing main README. Create circuits/README.md with major template paths and example include lines, then link it from the main README. Done means the listed paths match the tree and the index is reachable from the main README.

Written by the indexing model from the issue text.