fix: Cloudflare worker environment variable persistence and document CDN endpoint(s) (DIA-848)

[joeyAghion]

This adds basic documentation of the alternate, edge-cached Metaphysics endpoints.

Important bonus: fix the wrangler deploy command to avoid wiping out configured environment variables upon deployments (as discussed in https://github.com/cloudflare/workers-sdk/issues/276).

[mzikherman]

Thanks for this!