docs: update README and API documentation for clarity and accuracy

[ronibhakta1]

Closes #9
This pull request updates the README.md with clarifications and corrections regarding feature support, error handling, deployment details, and configuration options. The changes aim to provide a more accurate and comprehensive overview of the current state of the project, especially around word boundary support, error response formats, production deployment, and environment variables.

Feature Support and Status:

• Clarified that word boundary data is not currently populated by any provider, and all voices report boundary: false (README.md) [1] [2].
• Updated the provider status table to use "Planned" instead of "Coming soon" for Kokoro, ElevenLabs, and Azure Speech (README.md).

Deployment and Production Details:

• Expanded production deployment instructions to specify nginx behavior, rate limiting, port usage, and restart policies for containers (README.md).

Error Handling:

• Updated error response documentation to use RFC 9457 Problem Details format, and clarified which errors come from the app vs. nginx. Provided a more detailed error table and referenced a full API documentation file (README.md).

Configuration and Environment Variables:

• Clarified that API key enforcement is reserved and not yet active, and documented new environment variables for provider selection and domain configuration (README.md).