Add the sag ui command to the landing page

Add a third hero step (Open the Web UI -> `sag ui --port 8765`) with a
short note that the Web UI is one command away. The COPY button is picked
up generically by the existing clipboard handler.

Author: Codegass

website/css/style.css

@@ -186,6 +186,14 @@ header.g {
 
 .install-step { display: flex; flex-direction: column; gap: 4px; }
 
+.install-note {
+  margin: 0;
+  font-size: 0.8125rem;
+  line-height: 1.6;
+  color: var(--fg-dim);
+  font-weight: 300;
+}
+
 .install-head {
   display: flex;
   align-items: center;

website/index.html

@@ -48,6 +48,14 @@ <h2>Setup<br>Agent</h2>
           </div>
           <div class="cmd"><code><span>sag project https://github.com/apache/commons-cli.git</span></code></div>
         </div>
+        <div class="install-step">
+          <div class="install-head">
+            <small>3. Open the Web UI</small>
+            <button class="cp">COPY</button>
+          </div>
+          <div class="cmd"><code><span>sag ui --port 8765</span></code></div>
+        </div>
+        <p class="install-note">Now you can enjoy the Web UI with one command. Easier to use.</p>
       </div>
     </div>
   </div>