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.

src/config.rs

@@ -20,6 +20,40 @@ pub struct SiteConfig {
     /// Navigation configuration.
     #[serde(default)]
     pub nav: NavConfig,
+    /// Feed (Atom) configuration.
+    #[serde(default)]
+    pub feed: FeedConfig,
+    /// Sitemap configuration.
+    #[serde(default)]
+    pub sitemap: SitemapConfig,
+}
+
+/// Feed (Atom) generation configuration.
+#[derive(Debug, Deserialize)]
+#[serde(default)]
+pub struct FeedConfig {
+    /// Whether to generate an Atom feed (default: true).
+    pub enabled: bool,
+}
+
+impl Default for FeedConfig {
+    fn default() -> Self {
+        Self { enabled: true }
+    }
+}
+
+/// Sitemap generation configuration.
+#[derive(Debug, Deserialize)]
+#[serde(default)]
+pub struct SitemapConfig {
+    /// Whether to generate a sitemap.xml (default: true).
+    pub enabled: bool,
+}
+
+impl Default for SitemapConfig {
+    fn default() -> Self {
+        Self { enabled: true }
+    }
 }
 
 /// Navigation configuration.