docs: Document the url: import prefix for remote code

docs/guide/remote-code.md

@@ -0,0 +1,31 @@
+# Remote Code
+
+WXT will automatically download and bundle imports with the `url:` prefix so the extension does not depend of remote code, [a requirement from Google for MV3](https://developer.chrome.com/docs/extensions/migrating/improve-security/#remove-remote-code).
+
+## Google Analytics
+
+For example, you can import google analytics:
+
+```ts
+// utils/google-analytics.ts
+import 'url:https://www.googletagmanager.com/gtag/js?id=G-XXXXXX';
+
+window.dataLayer = window.dataLayer || [];
+// NOTE: This line is different from Google's documentation
+window.gtag = function () {
+  dataLayer.push(arguments);
+};
+gtag('js', new Date());
+gtag('config', 'G-XXXXXX');
+```
+
+Then you can import this in your HTML files to enable Google Analytics:
+
+```ts
+// popup/main.ts
+import '@/utils/google-analytics';
+
+gtag('event', 'event_name', {
+  key: 'value',
+});
+```