Fix documentation URL in README

[themisjia566-cell]

Corrects the Anthropic documentation link in README.md from docs.claude.com to docs.anthropic.com for the Managed Agents API reference.

[github-actions]

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• Opener: @themisjia566-cell
• Commit authors: @trae, @claude Opus 4.7, @claude Opus 4.8, @kanekanefy, @kanekanefy, @icedfish, @themisjia566-cell

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0 out of 6 committers have signed the CLA.
❌ @themisjia566-cell
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❌ @kanekanefy
❌ @kanekanefy
❌ @icedfish
❌ @themisjia566-cell

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