docs: address review feedback for Experiment Center example

- Simplify to single example with all 3 params (experiment_id,
  variation_id, segment_id) across Kotlin/coroutine/Java
- Add param descriptions and mention segment_id in intro
- Add Beta + development-tenant limitation to NOTE callout
- Add Experiment Center section to README.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>