Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

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

@@ -0,0 +1,14 @@
+---
+title: "MQTT Server API reference"
+symbol_kind: "intro"
+decl_name: "mqtt_server.h"
+items:
+  - { name: mg_mqtt_broker_init.md }
+  - { name: mg_mqtt_broker.md }
+  - { name: mg_mqtt_next.md }
+  - { name: struct_mg_mqtt_session.md }
+  - { name: struct_mg_mqtt_broker.md }
+---
+
+
+