Add example navigation menu for dropdown product menu

This is for:

- #176

But this is still just proof of concept level so far, the mobile menu is
a bit broken by this and the actual structure needs shaping up still.

readthedocs_theme/templates/includes/topnav.html

@@ -1,6 +1,47 @@
 {% macro menu_main() %}
-  <a class="item{% if page and page.slug == 'features' %} active" aria-current="page{% endif %}" href="{{ SITEURL }}/features/">Features</a>
-  <a class="item{% if page and page.slug == 'pricing' %} active" aria-current="page{% endif %}" href="{{ SITEURL }}/pricing/">Pricing</a>
+  <div
+      class="ui wide dropdown item{% if page and page.slug == 'features' %} active"
+      aria-current="page{% endif %}"
+      href="{{ SITEURL }}/features/"
+      data-module="dropdown"
+      data-module-on="hover">
+    Product
+    <div class="menu">
+
+      <div class="header">About</div>
+      <a class="item" href="/features">How Read the Docs works</a>
+      <div class="item">
+        Features
+        <i class="dropdown fad fa-caret-right icon"></i>
+        <div class="menu">
+          <a class="item" href="#">Building</a>
+          <a class="item" href="#">Hosting</a>
+          <a class="item" href="#">Authentication</a>
+          <a class="item" href="#">Management</a>
+        </div>
+      </div>
+
+      <div class="item">
+        Authoring tools
+        <i class="dropdown fad fa-caret-right icon"></i>
+        <div class="menu">
+          <a class="item" href="#">Sphinx</a>
+          <a class="item" href="#">Mkdocs</a>
+          <a class="item" href="#">Custom tools</a>
+        </div>
+      </div>
+
+      <a class="item" href="/choosing-a-platform/">Choosing a platform</a>
+
+      <div class="divider"></div>
+      <div class="header">Solutions</div>
+      <a class="item" href="#">For enterprise</a>
+      <a class="item" href="#">For engineering teams</a>
+    </div>
+  </div>
+  <a class="item{% if page and page.slug == 'pricing' %} active" aria-current="page{% endif %}" href="{{ SITEURL }}/pricing/">
+    Pricing
+  </a>
 {% endmacro %}
 
 {% macro menu_help() %}