docs: restore ProGuard setup instructions - #2171

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Fixes #2089 ☕️

Summary

Restores the missing ProGuard section in the setup documentation using the existing rules from google-http-client-assembly/proguard-google-http-client.txt.

The documentation also separates the Android-specific warning suppression from the common rules, making the instructions applicable to desktop applications.

Validation

• git diff --check
• Documentation-only change; Java tests were not required