Replace hardcoded "/blog/{slug}.html" URL pattern with dynamic
path derivation using Content.output_path(). This ensures feed
URLs work correctly for any content location, not just blog posts.
- src/feed.rs: New module with generate_atom_feed() that produces
Atom 1.0 XML from blog posts. Uses config.base_url for absolute
entry URLs. Includes xml_escape() helper.
- src/main.rs: Wire mod feed and call generate_feed() after blog
index generation. Outputs to public/feed.xml.
- src/templates.rs: Add <link rel="alternate" type="application/atom+xml">
autodiscovery link to page head using config.base_url.
Feed includes title, author, updated timestamp, and entries with
title, link, id, updated, and summary for each blog post.
