sync repo source code with rel-1.15.0 and update yml files into two stages

objectivec/docs/jazzy_config.yaml

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+module: ONNX Runtime Objective-C API
+author: ONNX Runtime Authors
+author_url: https://www.onnxruntime.ai
+github_url: https://github.com/microsoft/onnxruntime
+
+objc: true
+umbrella_header: ../include/onnxruntime.h
+framework_root: ..
+
+readme: ./main_page.md
+
+hide_documentation_coverage: true
+undocumented_text: ""

objectivec/docs/main_page.md

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+# ONNX Runtime Objective-C API
+
+ONNX Runtime provides an Objective-C API.
+
+It can be used from Objective-C/C++ or Swift with a bridging header.

objectivec/docs/readme.md

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+# Objective-C API Documentation
+
+The API should be documented with comments in the [public header files](../include).
+
+## Documentation Generation
+
+The [Jazzy](https://github.com/realm/jazzy) tool is used to generate documentation from the code.
+
+To generate documentation, from the repo root, run:
+
+```bash
+jazzy --config objectivec/docs/jazzy_config.yaml --output <output directory>
+```
+
+The generated documentation website files will be in `<output directory>`.
+
+[main_page.md](./main_page.md) contains content for the main page of the generated documentation website.