Document the public C API

[unamedkr]

The main public header include/turboquant/tq_engine.h exposes ~52 public functions but has minimal documentation. New users and contributors need a reference to understand the API surface.

What to do:

• Create docs/api.md listing each public function with its signature and a one-line description.
• Group functions by category (init/free, model loading, inference, KV cache, quantization).
• Optionally add a "Quick Start" section showing the minimal code to load a model and run inference.

Files to touch: docs/api.md (new), reference include/turboquant/tq_engine.h and include/turboquant/tq_types.h.