Fix render breakage introduced in #665 (#685)

A bare bones setup of two pages and two posts gets broken because of a
template change missed in #665

Fatal error message in readable form:
```
ERROR render:
failed to render pages:
render of "home" failed: "/themes/ananke/layouts/index.html:48:23":
execute of template failed:
template:
index.html:48:23:
executing "main" at <.Site.GetPage>: can't evaluate field Site in type string
```

Steps to reproduce:

```shell
hugo new site quickstart
cd quickstart
git init
git submodule add https://github.com/theNewDynamic/gohugo-theme-ananke.git themes/ananke
echo "theme = 'ananke'" >> hugo.toml
echo -e "+++\ntitle='Home'\n+++\n\nMain page" > content/_index.md
echo Foo. > content/page1.md
echo Bar > content/page2.md
mkdir -p content/post
echo "First post." > content/post/post1.md
echo "Second post." > content/post/post2.md
hugo server
Watching for changes in .../quickstart/{archetypes,assets,content,data,i18n,layouts,static,themes}
Watching for config changes in .../quickstart/hugo.toml, ...quickstart/themes/ananke/config.yaml
Start building sites …
hugo v0.125.3-474c4c02212cf97712c6fbf4159c68822ea6e078+extended darwin/amd64 BuildDate=2024-04-22T17:18:35Z VendorInfo=brew

Built in 67 ms
Error: error building site: render: failed to render pages: render of "home" failed: ".../quickstart/themes/ananke/layouts/index.html:48:23": execute of template failed: template: index.html:48:23: executing "main" at <.Site.GetPage>: can't evaluate field Site in type string
```

assets/ananke/css/_styles.css

@@ -18,3 +18,23 @@
 .nested-links a{
   overflow-wrap: break-word;
 }
+
+.menu-btn {
+  display: none; 
+  color: white;
+  font-size: 2em;
+}
+.nav-links {
+  display: flex;
+  justify-content: space-between;
+}
+
+@media (max-width: 600px) {
+  .nav-links {
+    display: none;
+    flex-direction: column;
+  }
+  .menu-btn {
+    display: block;
+  }
+}

layouts/partials/site-navigation.html

@@ -10,15 +10,39 @@
     <div class="flex-l items-center">
       {{ partial "i18nlist.html" . }}
       {{ if .Site.Menus.main }}
-        <ul class="{{ cond (eq $.Site.Language.LanguageDirection "rtl") "pr0 ml3" "pl0 mr3" }}">
-          {{ range .Site.Menus.main }}
-          <li class="list f5 f4-ns fw4 dib {{ cond (eq $.Site.Language.LanguageDirection "rtl") "pl3" "pr3" }}">
-            <a class="hover-white no-underline white-90" href="{{ .URL }}" title="{{ i18n "pageTitle" . }}">
+        <nav>
+          <div class="menu-btn">
+            <div class="dib">☰ Menu</div>
+          </div>
+          <div class="nav-links">
+            {{ range .Site.Menus.main }}
+            <a class="link hover-white white-90 no-underline f5 f4-ns {{ cond (eq $.Site.Language.LanguageDirection "rtl") "pl3" "pr3" }} " href="{{ .URL }}" title="{{ i18n "pageTitle" . }}">
               {{ .Name }}
             </a>
-          </li>
-          {{ end }}
-        </ul>
+            {{ end }}
+          </div>
+        </nav>
+
+        <script>
+          var navLinks = document.querySelector('.nav-links');
+          document.querySelector('.menu-btn').addEventListener('click', function() {
+            if (navLinks.style.display === 'none') {
+              navLinks.style.display = 'flex';
+            } else {
+              navLinks.style.display = 'none';
+            }
+          });
+       
+          // Adjust menu when window is resized
+          window.addEventListener('resize', function() {
+            if (window.innerWidth > 600) {
+              navLinks.style.display = 'flex';
+            } else {
+              navLinks.style.display = 'none';
+            }
+          });
+        </script>
+
       {{ end }}
       {{ partialCached "social-follow.html" . }}
     </div>