docs: lift user-facing guides to recipes/, drop dangling docs/ refs

docs/ is gitignored; references to docs/implementation/*.md from public
source, configs, and packaging were dead links outside the maintainer
machine. Adds four recipes (README, dnsdist-front, doh-on-lan,
odoh-upstream) under top-level recipes/ and repoints existing pointers.

- numa.toml, packaging/client/{README.md,numa.toml}: point to
  recipes/odoh-upstream.md.
- src/{bootstrap_resolver,forward,serve}.rs: reference issue #122
  directly (module scope is broader than the ODoH-specific recipe).
- src/health.rs: drop the §-ref; iOS HealthInfo remains named as the
  canonical consumer.

recipes/README.md

@@ -0,0 +1,11 @@
+# Recipes
+
+Scenario-driven configs for common Numa deployments. Each recipe is self-contained: copy the snippet, adjust the marked fields, reload.
+
+## Transport / encryption
+
+- [DoH on the LAN](doh-on-lan.md) — expose Numa's built-in DNS-over-HTTPS to local clients.
+- [dnsdist in front of Numa](dnsdist-front.md) — terminate public TLS externally, keep Numa on loopback.
+- [ODoH upstream with bootstrap pinning](odoh-upstream.md) — oblivious DNS client mode without leaking the relay/target hostnames.
+
+Missing a scenario? Open an issue or PR — these are plain Markdown with no build step.