docs: add custom preset instructions

docs/content/2.deploy/3.custom-presets.md

@@ -5,7 +5,77 @@ icon: ri:file-code-line
 # Custom Preset
 
 ::alert{type="warning"}
-**Work in Progress**
+**Advanced**
 :br
-Documentation for this section is not yet complete.
+This is an advanced feature.
 ::
+
+If you want to use a provider that Nitro doesn't support, or want to modify an existing one, you can create a custom preset.
+
+To create a custom preset, you need to use the `defineNitroPreset` helper, and specify a runtime entry point.
+
+```ts [custom-preset.ts]
+import type { NitroPreset } from "nitropack";
+import { fileURLToPath } from "node:url"
+
+export default <NitroPreset>{
+  extends: "node-server", // You can extend existing presets
+  entry: fileURLToPath(new URL("./custom-entry.ts", import.meta.url)),
+  hooks: {
+    compiled() {
+      console.info("Using Custom Preset!");
+    },
+  },
+};
+```
+
+In your entry point, use the virtual files to access the Nitro App.
+The entry point will be used by your server or provider, and you can fully customise its behaviour.
+
+```ts [custom-entry.ts]
+import "#internal/nitro/virtual/polyfill";
+const nitroApp = useNitroApp();
+
+// This syntax would be used for a cloudflare worker with the module syntax.
+export default {
+  async fetch(
+    request: Request,
+    env: CFModuleEnv,
+    context: ExecutionContext
+  ) {
+    console.log("This is a custom preset !")
+    //const response = await nitroApp.localFetch(...) <- Use localFetch to access your nitro application.
+    // ...
+  }
+  async queue(){
+    // queue logic 
+  }
+};
+```
+
+Then in your nitro config file, you can use your custom preset.
+
+::code-group
+```ts [nitro.config.ts]
+import { defineNitroConfig } from "nitropack/config";
+export default defineNitroConfig({
+  preset: "./custom-preset",
+});
+```
+```ts [nuxt.config.ts]
+export default defineNuxtConfig({
+  nitro: {
+    preset: "./custom-preset",
+  }
+});
+```
+::
+
+You an use the official Nitro [preset starter](https://github.com/unjs/nitro-preset-starter/blob/main/preset/nitro.config.ts) for a working example.
+
+Refer to the Nitro [source code](https://github.com/unjs/nitro/) directly to have a better understanding of presets and entry points.
+
+Here are two community examples of custom presets :
+
+- [Lambda-edge](https://github.com/jdevdevdev/nuxt-sst/blob/lambda-edge@%7B2023-07-03T11:20:03Z%7D/nuxt-app/server/nitro/preset/entry.ts)
+- [Fastify plugin](https://github.com/auguwu/fastify-nitro)