docs: add TTS server API documentation and link in README

[jeimison3]

This PR adds dedicated documentation for the TTS server component and links it from the main README.

Motivation:

• The TTS server provides an OpenAI-compatible HTTP API, but its usage details (endpoints, request parameters, response formats, and audio specifications) were not documented in user-facing materials.
• Users consuming the server via curl or client code had to infer behavior from source code or OpenAI conventions.

Changes:

• Added TTS_SERVER_README.md with endpoint documentation, request/response details, CLI arguments, and troubleshooting notes.
• Updated the main README.md to include a new "Server" section linking to the new documentation.

Note:

• This documentation was generated with assistance from Gemma4 26B, analyzing the implementation in tts-server.cpp and tts-server.h.

The goal is to make the server entry point clearer for integrators and remote consumers, without altering any code or behavior.