Allow document-specific public links within a publicly shared collection

Current Right now, publishing a collection to the web makes every document in it publicly visible, but only through the collection-level link — there's no way to get a public link that points directly to one specific document. Concretely: • Existing per-document links disappear. If a document already has its own public share link, publishing the parent collection causes that link to stop showing in the share dialog — it's replaced by the collection's link. • New per-document links can't be created either. After the collection is published, trying to share an individual document within it still only produces the collection-level link (which opens to the first document), not a link to that specific document. This works well for wiki-style collections where you want to hand someone the whole thing, but it means there's no way to send a direct link to a specific document (let alone a section of one — not supported yet either) once the parent collec…