docs: add documentation about the init method (#590)

Author: Jochum van der Ploeg

docs/docs/advanced/custom_dockerfile.md

@@ -1,5 +1,5 @@
 ---
-sidebar_position: 2
+sidebar_position: 3
 title: 🐳 Custom Dockerfile
 ---
 

docs/docs/advanced/custom_init_method.md

@@ -0,0 +1,35 @@
+---
+sidebar_position: 2
+title: 🛫 Custom Init Method
+---
+
+# Custom Init Method 🛫
+
+Dart Frog supports creating a custom entrypoint as shown in the [Custom Entrypoint docs](docs/advanced/custom_entrypoint) but that will run every time the server hot reloads. In cases where you want to initialize something only on server start, like setting up a database connection, you can use the `init` method.
+
+## Creating a Custom Init Method ✨
+
+To create a custom init method, simply create a `main.dart` file at the root of your Dart Frog project.
+
+:::warning
+Keep in mind that the `main.dart` file must expose a top-level `run` as mentioned in the [Custom Entrypoint docs](docs/advanced/custom_entrypoint).
+:::
+
+Add the following top-level `init` method to the `main.dart` file:
+
+```dart
+import 'dart:io';
+
+import 'package:dart_frog/dart_frog.dart';
+
+Future<void> init(InternetAddress ip, int port) async {
+  // Any code initialized within this method will only run on server start, any hot reloads
+  // afterwards will not trigger this method until a hot restart.
+}
+
+Future<HttpServer> run(Handler handler, InternetAddress ip, int port) {
+    ...
+}
+```
+
+The Dart Frog CLI will detect the `init` method and execute it on server start.

docs/docs/advanced/powered_by_header.md

@@ -1,5 +1,5 @@
 ---
-sidebar_position: 4
+sidebar_position: 5
 title: 🔋 Powered By Header
 ---
 

docs/docs/advanced/security_context.md

@@ -1,5 +1,5 @@
 ---
-sidebar_position: 5
+sidebar_position: 6
 title: 🔑 Security Context
 ---
 

docs/docs/advanced/web_socket.md

@@ -1,5 +1,5 @@
 ---
-sidebar_position: 3
+sidebar_position: 4
 title: 🔌 WebSockets
 ---