Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

docs/c-api/dns_server.h/intro.md

@@ -0,0 +1,12 @@
+---
+title: "DNS server API reference"
+symbol_kind: "intro"
+decl_name: "dns_server.h"
+items:
+  - { name: mg_dns_create_reply.md }
+  - { name: mg_dns_reply_record.md }
+  - { name: mg_dns_send_reply.md }
+---
+
+Disabled by default; enable with `-DMG_ENABLE_DNS_SERVER`.
+