fix(docs): add beta test join guidance to sidebar

Signed-off-by: Infi <infi@infi.sh>

docs/astro.config.mjs

@@ -33,6 +33,10 @@ export default defineConfig({
                         {
                             label: "Geographic Availability",
                             link: "/beta/availability-regions",
+                        },
+                        {
+                            label: "Join the Beta Test",
+                            link: "/beta/joining-the-public-beta-test",
                         }
                     ],
                 },