docs(overrides): add Asset Resolver (#168)

Author: vicb

pages/aws/config/custom_overrides.mdx

@@ -417,3 +417,21 @@ export default config;
 ```
 
 [Included](https://github.com/opennextjs/opennextjs-aws/tree/main/packages/open-next/src/overrides/proxyExternalRequest) ones are `'fetch' | 'node' | 'dummy'`
+
+## Custom Asset Resolver
+
+This is used by OpenNext to resolve static assets. You can read more about it [here](/aws/config/overrides/asset_resolver). To have a custom override for the Asset Resolver, you need an `open-next.config.ts` with this:
+
+```ts
+import type { OpenNextConfig } from "@opennextjs/aws/types/open-next.js";
+const config = {
+  default: {},
+  middleware: {
+    assetResolver: import("./customAssetResolver").then((mod) => mod.default),
+  },
+} satisfies OpenNextConfig;
+
+export default config;
+```
+
+[Included](https://github.com/opennextjs/opennextjs-aws/tree/main/packages/open-next/src/overrides/assetResolver) one is `'dummy'`

pages/aws/config/overrides/_meta.json

@@ -8,5 +8,6 @@
   "proxy_external_request": "External Request Proxy",
   "origin_resolver": "Origin Resolver",
   "invoke_function": "Invoke Function for the warmer",
-  "automatic_cdn_invalidation": "Automatic CDN Invalidation"
+  "automatic_cdn_invalidation": "Automatic CDN Invalidation",
+  "asset_resolver": "Asset Resolver"
 }

pages/aws/config/overrides/asset_resolver.mdx

@@ -0,0 +1,27 @@
+Available since `@opennextjs/aws` 3.7.0
+
+This override is used by OpenNext to serve [static assets](https://nextjs.org/docs/app/api-reference/file-conventions/public-folder) from the routing layer.
+
+It is not enabled by default as the assumption is that assets are served before reaching the Open Next server.
+
+When assets are not served before the Open Next server, you can create an Asset Resolver to serve them. It will be invoked [after the `beforeFiles` (`rewrites`) from `next.config.ts`](https://nextjs.org/docs/app/api-reference/file-conventions/middleware#execution-order).
+
+## Implementation
+
+An Asset Resolver should implement:
+
+```ts
+export interface AssetResolver {
+  name: string;
+
+  maybeGetAssetResult?: (event: InternalEvent) => Promise<InternalResult | undefined> | undefined;
+}
+```
+
+`maybeGetAssetResult` is invoked with the `InternalEvent` and should return a `Promise<InternalResult>` to serve an asset when the incoming event matches one. When it returns `Promise<undefined>` or `undefined`, the routing process keeps going to try and match Next routes.
+
+## Included Asset Resolver
+
+### dummy
+
+The dummy `AssetResolver` does nothing so it never serves assets. It is the default implementation.