docs: added documentation for Agent deployment on GKE cluster #445

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@vicentefb

The following contributors were found for this pull request:

✅ df063e3 Author: @vicentefb <vice*******ara@google.com>
✅ 0e74a29 Author: @hangfei <han*****in@gmail.com>
✅ 53153ef Author: @Sita04 <sit******mi@google.com>

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