Improve Sphinx documentation for PyQIR Generator

[bamarsha]

• Move the docstrings from .pyi to Rust doc comments, so PyO3 attaches them to the Python __doc__ attribute.
• Add parameter types and return types to the docstrings (this duplicates information still in the .pyi file).
• Add text_signature attributes.

This approach unfortunately has a lot of information duplication, but it seems like this is just where the state of the tooling is for creating Python native modules with good documentation. Some of the open issues in the toolchain:

• Autogenerate __text_signature__ PyO3/pyo3#310
• Feature request: Generate stub files along during compile PyO3/pyo3#510
• [Feature Request] add support for type annotation hints in #[text_signature] macro. PyO3/pyo3#1112
  • function annotation for builtin and C function python/cpython#47458
• Use PEP 484 stub files when documenting native extensions with autodoc sphinx-doc/sphinx#7630

Closes #62.