feat: normalize config context and template API

Add FeedConfig and SitemapConfig structs to config.rs
with enabled: bool (default true) for opt-out control.

Refactor ConfigContext: flat nested_nav bool replaced
with nested nav: NavContext { nested, toc } to mirror
site.toml [nav] table structure.

Remove standalone base_url template variable; use
config.base_url as single source of truth with
trailing-slash trimming in ConfigContext::from().

Add section template fallback: try section/<type>.html,
fall back to section/default.html for unknown types.

Delete section/features.html (duplicate of default.html)
and homepage.html (dead code, never referenced).

Update base.html for new variable names.

docs/templates/base.html

@@ -7,7 +7,7 @@
   {% if page.description %}
   <meta name="description" content="{{ page.description }}" />{% endif %}
   <title>{{ title }} | {{ config.title }}</title>
-  <link rel="canonical" href="{{ base_url }}{{ page_path }}" />
+  <link rel="canonical" href="{{ config.base_url }}{{ page_path }}" />
   <link rel="stylesheet" href="{{ prefix }}/style.css" />
   <link rel="icon" type="image/png" href="{{ prefix }}/logo.png" />
 </head>
@@ -39,7 +39,7 @@
         {% endfor %}
       </div>
       {% endif %}
-      {% if config.nested_nav and item.children %}
+      {% if config.nav.nested and item.children %}
       <div class="nav-children{% if is_current_section %} expanded{% endif %}">
         {% for child in item.children %}
         <a href="{{ prefix }}{{ child.path }}" {% if page_path==child.path %}class="active" {% endif %}>{{ child.label

docs/templates/homepage.html

@@ -1,9 +0,0 @@
-{% extends "base.html" %} {% block content %}
-<article class="homepage">
-  <h1>{{ page.title }}</h1>
-  {% if page.description %}
-  <p class="lead">{{ page.description }}</p>
-  {% endif %}
-  <section class="content">{{ content | safe }}</section>
-</article>
-{% endblock content %}

docs/templates/section/features.html

@@ -1,18 +0,0 @@
-{% extends "base.html" %} {% block content %}
-<article class="section-index">
-  <h1>{{ section.title }}</h1>
-  {% if section.description %}
-  <p class="lead">{{ section.description }}</p>
-  {% endif %}
-  <nav class="section-nav">
-    {% for item in items %}
-    <a href="{{ prefix }}{{ item.path }}" class="section-link">
-      <strong>{{ item.frontmatter.title }}</strong>
-      {% if item.frontmatter.description %}
-      <span>{{ item.frontmatter.description }}</span>
-      {% endif %}
-    </a>
-    {% endfor %}
-  </nav>
-</article>
-{% endblock content %}