recipes doc (#1165)

### What does this PR do?

Added an extensive guide for the ModelOpt recipe system: recipe
structure, YAML schema (quantization-focused), built-in discovery and
path resolution, floating-point shorthand conversion, example usage
(Python/CLI), authoring guidance, repository layout, and future
directions.


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## Summary by CodeRabbit

* **Documentation**
* Added comprehensive guide for ModelOpt recipes, introducing
declarative YAML-based optimization specifications. Documents recipe
structure, configuration loading, path resolution, and the three-layer
system architecture. Includes built-in recipe discovery conventions and
detailed instructions for authoring custom recipes with practical
examples.

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Signed-off-by: Shengliang Xu <shengliangx@nvidia.com>

Author: shengliangxu