Document Cloudflare Workers Paid plan requirement for Cloudflare Sandbox

[astrobot-houston]

Originally proposed by @Zewang0217 in #585. Their description is reproduced verbatim below.

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Closes #581

Adds a warning callout to the Cloudflare Sandbox docs stating that deploying Cloudflare Sandbox requires the Cloudflare Workers Paid plan, with a link to Cloudflare pricing. Distinguishes local Docker-based development (not subject to the requirement) from deployed usage.

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Original implementation from #585 by @Zewang0217