Discover public sites on letter.pub

Browse an index of public Letter.md sites and open each publication at its canonical address.

Public Letter.md sites are now easier to discover from one quiet directory.

What changed

letter.pub lists sites that have at least one indexable public document.
Site cards link directly to the publisher's canonical address.
Photo notes can appear as visual entries without changing their source Markdown.
Public feeds, archives, and site introductions help readers understand each collection.
Sites with no indexable content stay out of discovery.

Authors still control whether each publication is allowed to appear in search and discovery.