docs: extend example with a before hook

Author: hassy

examples/browser-load-testing-playwright/browser-load-test.ts

@@ -13,6 +13,14 @@ export const config = {
   }
 };
 
+export const before = {
+  engine: 'playwright',
+  testFunction: async function beforeFunctionHook(_page, userContext, _events) {
+    // Any scenario variables we add via userContext.vars in this before hook will be available in every VU
+    userContext.vars.testStartTime = new Date();
+  }
+};
+
 export const scenarios = [
   {
     engine: 'playwright',