docs: add custom preset instructions

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

@@ -5,7 +5,98 @@ icon: ri:file-code-line
 # Custom Preset
 
 ::alert{type="warning"}
-**Work in Progress**
-:br
-Documentation for this section is not yet complete.
+Custom local preset support is an experimental feature.
 ::
+
+If you want to use a provider that Nitro doesn't support, or want to modify an existing one, you can create a local custom preset in your project.
+
+Custom presets are local files that have a preset entry that defines builder configuration and a runtime entry point.
+
+## Example
+
+::alert
+Check [unjs/nitro-preset-starter](https://github.com/unjs/nitro-preset-starter) for a ready-to-use template.
+::
+
+First, we have to define our preset entry point in a local directory `preset/index.ts`
+
+```ts [./preset/index.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("./entry.ts", import.meta.url)),
+  hooks: {
+    compiled() {
+      // ...
+    },
+  },
+};
+```
+
+The entry point will be used by your server or provider, and you can fully customize its behavior.
+
+::code-group
+```ts [preset/entry.ts (Workers)]
+import "#internal/nitro/virtual/polyfill";
+
+const nitroApp = useNitroApp();
+
+export default {
+  fetch(request: Request) {
+    const url = new URL(request.url);
+    return nitroApp.localFetch(url.pathname + url.search, {
+      context: {},
+      host: url.hostname,
+      protocol: url.protocol,
+      method: request.method,
+      headers: request.headers,
+      body: undefined,
+    });
+  },
+};
+```
+
+```ts [preset/entry.ts (Node.js)]
+import "#internal/nitro/virtual/polyfill";
+import { Server } from "node:http";
+import { toNodeListener } from "h3";
+
+const nitroApp = useNitroApp();
+const server = new Server(toNodeListener(nitroApp.h3App));
+
+// @ts-ignore
+server.listen(3000, (err) => {
+  if (err) {
+    console.error(err);
+    process.exit(1);
+  }
+  console.log(`Listening on http://localhost:3000 (custom preset)`);
+});
+```
+::
+
+
+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: "./preset",
+});
+```
+
+```ts [nuxt.config.ts]
+export default defineNuxtConfig({
+  nitro: {
+    preset: "./preset",
+  }
+});
+```
+::
+
+
+Refer to the Nitro [source code](https://github.com/unjs/nitro/tree/main/src) directly to have a better understanding of presets and entry points.