Add image base path configuration option (#23115)

I similarly [couldn't find this info](https://github.com/vercel/next.js/issues/19711#issuecomment-750414110) in the docs about configuring the image path when using an app `basePath`, so thought I'd add it.

docs/basic-features/image-optimization.md

@@ -94,6 +94,18 @@ If you need a different provider, you can use the [`loader`](/docs/api-reference
 
 > The `next/image` component's default loader is not supported when using [`next export`](/docs/advanced-features/static-html-export.md). However, other loader options will work.
 
+### Base path
+
+If you have deployed your application under a sub-path of a domain using the [`basePath`](/docs/api-reference/next.config.js/basepath.md) option, you will need to configure your images to reference this path.
+
+```js
+module.exports = {
+  images: {
+    path: `yourBasePath/_next/image`,
+  },
+}
+```
+
 ## Caching
 
 The following describes the caching algorithm for the default [loader](#loader). For all other loaders, please refer to your cloud provider's documentation.