Create an event tarball

We currently produce a consolidated "book" or proceedings, but are going to need to stop. For large conferences these books are approaching 10k pages and are pushing the bounds of what can be handled in the PDF format. At the same time, the book is useful for offline browsing of a proceedings. It would be nice if we provided the option to download a tarball for an entire event—basically, everything linked to from the event page (e.g., https://aclanthology.org/events/acl-2023), including PDFs.