cmd/hello: link to the Hello KB article (#11022)

Fixes https://github.com/tailscale/corp/issues/17104

Signed-off-by: Chris Palmer <cpalmer@tailscale.com>

cmd/hello/hello.tmpl.html

@@ -430,6 +430,8 @@
     <footer class="footer text-gray-600 text-center mb-12">
       <p>Read about <a href="https://tailscale.com/kb/1017/install#advanced-features" class="text-blue-600 hover:text-blue-800"
           target="_blank">what you can do next &rarr;</a></p>
+      <p>Read about <a href="https://tailscale.com/kb/1073/hello" class="text-blue-600 hover:text-blue-800"
+          target="_blank">the Hello service &rarr;</a></p>
     </footer>
   </main>
 </body>