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.

packaging/client/numa.toml

@@ -1,7 +1,7 @@
 # Numa — ODoH client mode (docker-compose starter).
 # Sends every DNS query through an independent relay + target pair so
 # neither operator sees both your IP and your question. See
-# docs/implementation/odoh-integration.md for the protocol details and
+# recipes/odoh-upstream.md for the protocol details and
 # packaging/client/README.md for deploy notes.
 
 [server]