UI Navigation: Link to project category.

Without it, going from one project to another in the same category (like
going from Textures library to HDRI) is cumbersome, having to go always
through the homepage first.

src/templates/_macros/_navigation.pug

@@ -125,6 +125,12 @@ button.navbar-toggler(
 		i.pi-blender-cloud
 
 	| {% if project.url != 'blender-cloud' %}
+	+nav-secondary-link(
+		href="{{ category_url }}",
+		class="px-0")
+		span {{ category_title }}
+	li(class="nav-item px-1")
+		i.pi-angle-right
 
 	+nav-secondary-link(
 		class="font-weight-bold{% if title == 'landing' %} active{% endif %}",