Evaluate cache headers

[duckduckgrayduck]

Audit cache headers for API responses on /api/documents/

[duckduckgrayduck]

It seems we do not set a cache header on the viewer? Cloudflare should implicitly cache it, but for some reason this fails at times and seems to be set my location. It can create cases like this. We should add a cache header along with versioning
to avoid extraneous requests to origin and to avoid old viewer from being cached when we deploy:

[eyeseast]

Almost all of those are going to the legacy embed script: https://www.documentcloud.org/viewer/loader.js. That's being served by Cloudflare's static file handling, so it's fine.

[eyeseast]

Research spike: https://github.com/MuckRock/research/blob/main/cache-older-documents/api-recommendations.md

I'm going to make new issues based on this.