Make the navbar non-mandatory

I can imagine duckquill being used for one-page sites that simply don't need it, plus we can get rid of one more variable from theme.toml

templates/partials/nav.html

@@ -1,3 +1,4 @@
+{% if config.extra.nav.links %}
 <header id="site-nav">
 	<nav>
 		<a href="#main" id="main-content" tabindex="0">Skip to main content</a>
@@ -27,3 +28,4 @@
 		</ul>
 	</nav>
 </header>
+{% endif %}