Support multiple feed-level links and emit Atom rel="self" #65

Problem

Feed-level links are currently modeled as a single value (setLink()), which makes it hard to represent both:

1. Homepage link (rel="alternate")
2. Canonical feed URL (rel="self")

Atom consumers and validators commonly expect a self link for the feed.

Expected behavior

1. First-class support for both homepage URL and feed URL.
2. Support multiple feed-level links with relation metadata.
3. Atom output should emit at least one rel="self" link when feed URL is known.

Example Atom output

<feed xmlns="http://www.w3.org/2005/Atom">
  <title>Example Feed</title>
  <link rel="alternate" href="https://example.com/" type="text/html"/>
  <link rel="self" href="https://example.com/feed.atom" type="application/atom+xml"/>
</feed>

Suggested API

$feed->setHomePageUrl('https://example.com/');
$feed->setFeedUrl('https://example.com/feed.atom');

$feed->addLink('alternate', 'https://example.com/', 'text/html');
$feed->addLink('self', 'https://example.com/feed.atom', 'application/atom+xml');

Backward compatibility

1. Keep setLink() behavior unchanged for existing users.
2. Prefer explicit link collection when new API is used.