Add recipes system documentation guide

New _recipes.rst covers recipe file format, built-in recipes, loading
API, ExMy notation, path resolution, and future directions.

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

Author: shengliangxu

docs/source/guides/1_quantization.rst

@@ -20,6 +20,7 @@ Below, you can find the documentation for the quantization toolkit in ModelOpt:
     ./_choosing_quant_methods.rst
     ./_pytorch_quantization.rst
     ./_quant_cfg.rst
+    ./_recipes.rst
     ./_customized_model_quantization.rst
     ./_compress_quantized_models.rst
     ./_onnx_quantization.rst