[v6]: add docs for entryType in Adapter API

src/content/docs/en/reference/adapter-reference.mdx

@@ -246,6 +246,49 @@ export default function createIntegration() {
 }
 ```
 
+### `entryType`
+
+<p>
+
+**Type:** `"legacy-dynamic" | "self"`<br />
+**Default**: `"legacy-dynamic"`<br />
+<Since v="6.0.0" />
+</p>
+
+Specifies whether the adapter's entrypoint is automatically created by Astro (default), or whether a custom entrypoint is used.
+
+To provide your own entrypoint, configure `entryType: "self"` and set [`rollupOptions.input`](https://rollupjs.org/configuration-options/#input) with your custom entrypoint path.
+
+The following example defines the `entryType` and Rollup options to tell Astro that a custom entrypoint is provided:
+
+```js title="my-adapter.mjs" {9-11,20}
+export default function createIntegration() {
+  return {
+    name: '@example/my-adapter',
+    hooks: {
+      'astro:config:setup': ({ updateConfig }) => {
+        updateConfig({
+          vite: {
+            build: {
+              rollupOptions: {
+                input: "@example/my-adapter/custom-entrypoint.js"
+              }
+            }
+          }
+        })
+      },
+      'astro:config:done': ({ setAdapter }) => {
+        setAdapter({
+          name: '@example/my-adapter',
+          serverEntrypoint: '@example/my-adapter/server.js',
+          entryType: "self",
+        });
+      },
+    },
+  };
+}
+```
+
 ## Building a server entrypoint
 
 You will need to create a file that executes during server-side requests to enable on-demand rendering with your particular host. Astro's adapter API attempts to work with any type of host and gives a flexible way to conform to the host APIs.