Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

docs/overview/concept.md

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+---
+title: Design Concept
+---
+
+Mongoose has three basic data structures:
+
+- `struct mg_mgr` is an event manager that holds all active connections
+- `struct mg_connection` describes a connection
+- `struct mbuf` describes data buffer (received or sent data)
+
+Connections could be either *listening*, *outbound* or *inbound*. Outbound
+connections are created by the `mg_connect()` call. Listening connections are
+created by the `mg_bind()` call. Inbound connections are those accepted by a
+listening connection. Each connection is described by the `struct mg_connection`
+structure, which has a number of fields like socket, event handler function,
+send/receive buffer, flags, etc.
+
+An application that uses mongoose should follow a standard pattern of
+event-driven application:
+
+1. declare and initialise event manager:
+
+    ```c
+    struct mg_mgr mgr;
+    mg_mgr_init(&mgr, NULL);
+    ```
+2. Create connections. For example, a server application should create
+   listening connections:
+
+   ```c
+    struct mg_connection *c = mg_bind(&mgr, "80", ev_handler_function);
+    mg_set_protocol_http_websocket(c);
+   ```
+
+3. create an event loop by calling `mg_mgr_poll()` in a loop:
+
+    ```c
+    for (;;) {
+      mg_mgr_poll(&mgr, 1000);
+    }
+    ```
+
+`mg_mgr_poll()` iterates over all sockets, accepts new connections, sends and
+receives data, closes connections and calls event handler functions for the
+respective events. For the full example, see
+[Usage Example](#/overview/usage-example.md/)
+which implements TCP echo server.