Add ONBOARDING.md for java-storage

[nidhiii-27]

This PR adds a comprehensive onboarding guide (ONBOARDING.md) for the google-cloud-storage library. It covers the public API surface, implementation details, and traces execution paths for key operations down to the low-level HTTP/gRPC calls. This guide is intended to help new developers understand the codebase and architecture.

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PR created automatically by Jules for task 3157172586432514677 started by @nidhiii-27

[google-labs-jules]

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