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.
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