docs: custom server entrypoint (#274)

Author: felangel

docs/docs/advanced/_category_.json

@@ -0,0 +1,9 @@
+{
+  "label": "Advanced",
+  "position": 5,
+  "link": {
+    "title": "Advanced 🧙",
+    "type": "generated-index",
+    "description": "Learn advanced Dart Frog concepts."
+  }
+}

docs/docs/advanced/custom_entrypoint.md

@@ -0,0 +1,27 @@
+---
+sidebar_position: 1
+---
+
+# Custom Server Entrypoint 🎬
+
+Dart Frog supports creating a custom entrypoint in cases where you need fine-grained control over the server initialization or wish to execute code prior to starting the server.
+
+## Creating a Custom Entrypoint ✨
+
+To create a custom entrypoint, simply create a `main.dart` file at the root of your Dart Frog project. The `main.dart` file must expose a top-level `run` method with the following signature:
+
+```dart
+import 'dart:io';
+
+import 'package:dart_frog/dart_frog.dart';
+
+Future<HttpServer> run(Handler handler, InternetAddress ip, int port) {
+  // 1. Execute any custom code prior to starting the server...
+
+  // 2. Use the provided `handler`, `ip`, and `port` to create a custom `HttpServer`.
+  // Or use the Dart Frog serve method to do that for you.
+  return serve(handler, ip, port);
+}
+```
+
+The Dart Frog CLI will detect the custom entrypoint and execute your custom `run` method instead of the default implementation.