[Documentation] [Python API] Add documentation for onnxruntime module level functions

### Describe the documentation issue

These module level functions don't seem to be [documented](https://onnxruntime.ai/docs/api/python/api_summary.html#api-details).
https://github.com/microsoft/onnxruntime/blob/d2dfbf41795e72911643e2ffcadac069b72580bd/onnxruntime/__init__.py#L36-L48

Some are quite useful - e.g., `set_default_logger_severity` for setting the default log level.

### Page / URL

https://onnxruntime.ai/docs/api/python/api_summary.html

Start with onnxruntime/__init__.py lines 36-48 and compare those module-level functions with the Python API summary at https://onnxruntime.ai/docs/api/python/api_summary.html. Document the functions that are missing from the API details, including their useful behavior, and verify that the page exposes them clearly.

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