[Python] Improve Python stubs generation (#2606)

## Motivation and Context
This PR improves the Python stubs generation.

## Description
The main change is about avoiding to setup a placeholder for the Python
module and use the real module name, which allows to generate correct
docstrings during codegeneration.

We also change the stubs layout on disk, with the main stub entrypoint
called `__init__.pyi` instead of `$module_name.pyi`.

The README from the Rust runtime crate has been moved completely to the
example folder and I run autoformatting and style checks on the Python
example code.

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Signed-off-by: Bigo <1781140+crisidev@users.noreply.github.com>
Co-authored-by: Burak <unexge@gmail.com>