fix: added callbacks doc

docs/callbacks.md

@@ -0,0 +1,31 @@
+---
+title: Callbacks
+sidebar_label: Callbacks
+sidebar_position: 5
+---
+
+WebSerial comes with a single callback called `onMessage`. It's responsible for notifying your firmware whenever it has received a command/message from user via webserial terminal.
+
+If the callback is not registered, then the command received from terminal is simply discarded.
+
+### Example Usage
+
+```cpp
+...
+#include <WebSerial.h>
+
+void setup() {
+  ...
+
+  /* Attach Message Callback */
+  WebSerial.onMessage([&](uint8_t *data, size_t len) {
+    Serial.printf("Received %lu bytes from WebSerial: ", len);
+    Serial.write(data, len);
+    Serial.println();
+  });
+}
+
+void loop() {
+  ...
+}
+```