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.

numa.toml

@@ -51,8 +51,8 @@ api_port = 5380
 # relay_ip = "178.104.229.30"                   # optional: pin IPs so numa doesn't leak the
 # target_ip = "104.16.249.249"                  # relay/target hostnames via the bootstrap
 #                                                # resolver on cold boot when numa is its
-#                                                # own system DNS. See docs/implementation/
-#                                                # bootstrap-resolver.md.
+#                                                # own system DNS. See
+#                                                # recipes/odoh-upstream.md.
 # root_hints = [                                # only used in recursive mode
 #     "198.41.0.4",      # a.root-servers.net (Verisign)
 #     "199.9.14.201",    # b.root-servers.net (USC-ISI)