docs: add BYOVA productization and onboarding guidance

[adamweeks]

Summary

• refocus the root README on gateway purpose, ownership boundaries, and a concise local smoke test
• add customer evaluation, local development, JWT authentication, testing, and production-readiness guides
• document the complete BYOVA prerequisites for end-to-end testing and recommend the full Webex setup guide
• correct configuration and monitoring documentation so it reflects behavior implemented by the current sample

Why

The previous README mixed local setup, customer onboarding, authentication, monitoring, and production guidance into one long entry point. It also made the local quick start appear sufficient for an end-to-end BYOVA deployment. This split gives each audience a focused guide while making the sample-code and production-ownership boundary explicit.

User impact

New BYOVA evaluators can identify the required organization enablement, Service App authorization, public endpoint, data-source registration, and Contact Center flow before attempting a call. Developers retain a short local path, and teams productizing the sample gain a dedicated readiness checklist covering security, capacity, observability, alerting, incident response, and launch gates.

Validation

• git diff --cached --check
• verified local Markdown link targets across all changed documents
• xmllint --noout docs/images/byova-gateway-overview.drawio.svg
• documentation site build not run: the lockfile requires Bundler 2.6.9, which requires Ruby 3.1 or later, while the available system Ruby is 2.6.10

No Python runtime behavior changed.