feat(site): add healthz page (#458)

Summary:

This is a port of v1 healthz page to v2

Impact:

A simple way to check site health (requires no auth). Good smoke test
for e2e.

site/e2e/tests/healthz.spec.ts

@@ -0,0 +1,8 @@
+import { test } from "@playwright/test"
+import { HealthzPage } from "../pom/HealthzPage"
+
+test("Healthz is available without authentication", async ({ baseURL, page }) => {
+  const healthzPage = new HealthzPage(baseURL, page)
+  await page.goto(healthzPage.url, { waitUntil: "networkidle" })
+  await healthzPage.getOk().waitFor({ state: "visible" })
+})