Factor out common module prefix when all modules are under a same namespace.

crates/project/templates/_layout.html

@@ -185,6 +185,9 @@
         {% endif %}
 
         <h2>Modules</h2>
+	{% if !modules_prefix.is_empty() %}
+	<h3 class="modules-prefix">{{ modules_prefix }}/</h3>
+	{% endif %}
         <ul>
         {% for module in modules %}
           <li><a href="{{ breadcrumbs }}/{{ module.path }}">{{ module.name }}</a></li>

crates/project/templates/css/index.css

@@ -305,12 +305,21 @@ p code {
   margin: 0;
 }
 
+.sidebar h3.modules-prefix {
+  color: var(--pink);
+  margin-bottom: 0;
+}
+
 .sidebar ul {
   list-style: none;
   margin: var(--small-gap) 0;
   padding: 0;
 }
 
+.sidebar h3.modules-prefix + ul {
+  margin-left: var(--tiny-gap);
+}
+
 .sidebar li {
   line-height: 1.2;
   margin-bottom: 4px;