Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

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

@@ -0,0 +1,17 @@
+---
+title: "struct mg_mqtt_session"
+decl_name: "struct mg_mqtt_session"
+symbol_kind: "struct"
+signature: |
+  struct mg_mqtt_session {
+    struct mg_mqtt_broker *brk;          /* Broker */
+    struct mg_mqtt_session *next, *prev; /* mg_mqtt_broker::sessions linkage */
+    struct mg_connection *nc;            /* Connection with the client */
+    size_t num_subscriptions;            /* Size of `subscriptions` array */
+    struct mg_mqtt_topic_expression *subscriptions;
+    void *user_data; /* User data */
+  };
+---
+
+MQTT session (Broker side). 
+