Store local cache of Sessionize assets

[ewels]

Currently we get the URL to static assets (PDFs, speaker photos, custom share images etc) from Sessionize and use these directly in the HTML - effectively hotlinking.

It works fine, however these cache URLs are ephemeral and not guaranteed to be stable. To ensure stability, especially once we stop deploying on a regular basis, we should cache the assets in the build locally.

The best way to do this seems to be with content collections content layer API which was released in Astro 5. It should be possible to use this for static assets, see for example this.