docs-2978: fail the build on broken anchors

Set onBrokenAnchors to 'throw' in docusaurus.config.js so a broken #anchor
fails the build instead of being ignored.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ctauchen

docusaurus.config.js

@@ -37,7 +37,7 @@ export default async function createAsyncConfig() {
     tagline: 'Unified network security and observability for Kubernetes',
     url: 'https://docs.tigera.io',
     baseUrl: '/',
-    onBrokenAnchors: 'ignore',
+    onBrokenAnchors: 'throw',
     onBrokenLinks: 'throw',
     favicon: 'img/calico-logo-2026-badge.png',
     markdown: {