add table of contents to feature page

static/features.html

@@ -60,6 +60,16 @@
             AOSP and the hardware are not covered here. Documentation on that will be gradually
             added elsewhere on our site.</p>
 
+            <nav id="table-of-contents">
+                <h2><a href="#table-of-contents">Table of contents</a></h2>
+
+                <ul>
+                    <li><a href="#grapheneos">GrapheneOS</a></li>
+                    <li><a href="#services">Services</a></li>
+                    <li><a href="#project">Project</a></li>
+                </ul>
+            </nav>
+
             <section id="grapheneos">
                 <h2><a href="#grapheneos">GrapheneOS</a></h2>