docs: restore ProGuard setup instructions

[arnabnandy7]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

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