feat(docs): add Google Analytics (GTM + GA4) to Docusaurus site

Mirrors the pattern from Couchbase-Ecosystem/cbl-ionic-docs#51:
adds googleTagManager (GTM-MVPNN2) and gtag (G-J3B5K0YX8B) plugins
inside preset-classic options. The Docusaurus plugins handle SPA
navigations correctly out of the box.

Verified locally via Chrome DevTools: gtm.js + gtag/js load on initial
page, GA4 page_view beacon fires on initial load, and a new page_view
beacon fires on client-side navigation with the updated path.

Author: teetangh

website/docusaurus.config.js

@@ -47,6 +47,13 @@ const config = {
         theme: {
           customCss: "./src/css/custom.css",
         },
+        googleTagManager: {
+          containerId: "GTM-MVPNN2",
+        },
+        gtag: {
+          trackingID: "G-CVKKEY0D6B",
+          anonymizeIP: true,
+        },
       }),
     ],
   ],