chore: deploy static docs via docs-router-prod (R2 + KV) instead of CF Pages

[thesandlord]

Summary

Switches "Deploy Static Docs" from the container's Cloudflare Pages deploy to the docs-router publish path: the built dist/ goes to the fern-docs-static R2 bucket as an immutable build, and the docs-router-prod Worker's KV pointer is flipped at it. Going live is that single KV write, so the swap is atomic and a rollback is the same write aimed at an older build id.

The container picks the path off the env, so this is a swap of three vars:

-CF_PAGES_PROJECT / CF_PAGES_BRANCH        # legacy: wrangler pages deploy dist
+FERN_DOCS_STATIC_BUCKET: fern-docs-static # → sync-dist --build-id + KV pointer flip
+CLOUDFLARE_R2_ACCESS_KEY_ID / CLOUDFLARE_R2_SECRET_ACCESS_KEY

CLOUDFLARE_API_TOKEN keeps its role (now the KV pointer write rather than the Pages upload); R2 uploads are S3-signed, so they need their own key pair — hence the two new secrets, which must be created (R2 → Manage R2 API Tokens, Object Read & Write scoped to fern-docs-static) before the next run.

Container-side implementation: https://github.com/fern-api/fern-platform/pull/13948. That PR must merge and the fernenterprise/docs-static-build:latest image must be rebuilt before this workflow's new env has any effect — until then the added vars are inert and the build still deploys to Pages.

Link to Devin session: https://app.devin.ai/sessions/9924a72b0e59409a90883d7868a4e6b4
Requested by: @thesandlord

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