Fixes in MQTT for AWS support

PUBLISHED_FROM=2b82f3793b3c6d0cf1266e4cc0e67930f43002c5

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

@@ -20,6 +20,7 @@ items:
   - { name: mg_send_mqtt_handshake.md }
   - { name: mg_send_mqtt_handshake_opt.md }
   - { name: mg_set_protocol_mqtt.md }
+  - { name: struct_mg_mqtt_proto_data.md }
 ---
 
 

docs/c-api/mqtt.h/struct_mg_mqtt_proto_data.md

@@ -0,0 +1,12 @@
+---
+title: "struct mg_mqtt_proto_data"
+decl_name: "struct mg_mqtt_proto_data"
+symbol_kind: "struct"
+signature: |
+  struct mg_mqtt_proto_data {
+    uint16_t keep_alive;
+  };
+---
+
+mg_mqtt_proto_data should be in header to allow external access to it 
+