docs: refresh README for 1.0

[endrju19]

Summary

• restructure the README around a concise Spring Boot quick start and verified runtime behavior
• clarify delivery guarantees, retries, transactions, configuration, metrics, modules, and compatibility
• move the pre-1.0 migration procedure out of the README into UPGRADING.md
• add the Okapi module diagram from the SoftwareMill article

Verification

• git diff --check
• OkapiMicrometerAutoConfigurationTest (14 tests passed)
• migration instructions were not executed, as they are outside the scope of this documentation change