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/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 %}