Update README with concise summary and tailored usage information

[engine-labs-paul]

Summary

This PR updates the README to provide a concise summary of the tracking proxy, clarify its core functionality, and tailor quick-start and configuration instructions for this forked repository.

Details

• Rewrote README to succinctly describe the nginx proxy's function as an intermediary for Mixpanel tracking requests
• Added clear explanations of what endpoints are proxied and how geolocation/IP handling is supported
• Simplified and updated quick-start instructions, including Docker and cloud deployment usage for the fork
• Included notes about data residency configuration (US, EU, IN)
• Clarified that this repository is a fork of the original Mixpanel tracking-proxy

Warning: Task VM test is not fully passing, using partial setup (2/2 steps working)