feat: convert templates from njk to html

Note: Nunjucks will still process the templates, but this way they can
be plain HTML files and will be formatted properly.

src/_includes/partials/menu.njk

@@ -1,20 +0,0 @@
-<nav id="mainnav" class="navbar" aria-label="Main">
-  Menu.
-  <ul>
-    {% for item in navigation.top %}
-    <li>
-      <a
-        class="nav"
-        href="{{ item.url }}"
-        {{
-        helpers.getLinkActiveState(item.url,
-        page.url)
-        |
-        safe
-        }}
-        >{{ item.text }}</a
-      >
-    </li>
-    {% endfor %}
-  </ul>
-</nav>