Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

docs/overview/usage-example.md

@@ -0,0 +1,44 @@
+---
+title: Example - TCP echo server
+---
+
+- Copy `mongoose.c` and `mongoose.h` to your build tree
+- Write code that uses the Mongoose API, e.g. in `my_app.c`
+- Compile application: `$ cc my_app.c mongoose.c`
+
+```c
+#include "mongoose.h"  // Include Mongoose API definitions
+
+// Define an event handler function
+static void ev_handler(struct mg_connection *nc, int ev, void *ev_data) {
+  struct mbuf *io = &nc->recv_mbuf;
+
+  switch (ev) {
+    case MG_EV_RECV:
+      // This event handler implements simple TCP echo server
+      mg_send(nc, io->buf, io->len);  // Echo received data back
+      mbuf_remove(io, io->len);      // Discard data from recv buffer
+      break;
+    default:
+      break;
+  }
+}
+
+int main(void) {
+  struct mg_mgr mgr;
+
+  mg_mgr_init(&mgr, NULL);  // Initialize event manager object
+
+  // Note that many connections can be added to a single event manager
+  // Connections can be created at any point, e.g. in event handler function
+  mg_bind(&mgr, "1234", ev_handler);  // Create listening connection and add it to the event manager
+
+  for (;;) {  // Start infinite event loop
+    mg_mgr_poll(&mgr, 1000);
+  }
+
+  mg_mgr_free(&mgr);
+  return 0;
+}
+```
+