Merge and migrate Python API documents into docstring

[lan496]

This PR moves Python API documents into docstring of python/spglib/spglib.py. autodoc2 is used to generate API documents (which is compatible with myst). It will diminish the burden for document maintenance somewhat.

@atztogo Spglib uses numpy-style docstring, but autodoc2 does not support it. sphinx.ext.napoleon does not work as well. Do we use sphinx's python-domain directives instead? Autodoc2 seems to support MyST docstring with these directives.
https://sphinx-autodoc2.readthedocs.io/en/latest/quickstart.html#using-markdown-myst-docstrings

[lan496]

• Generated Python interface page: https://spglib--387.org.readthedocs.build/en/387/python-interface/python-interface.html
• API reference: https://spglib--387.org.readthedocs.build/en/387/python-interface/python-apidocs/spglib/spglib.html

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7aa3491) 83.86% compared to head (34fd27a) 83.86%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #387   +/-   ##
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  Coverage    83.86%   83.86%           
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  Files           24       24           
  Lines         8180     8180           
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  Hits          6860     6860           
  Misses        1320     1320           

Flag	Coverage Δ
c_api	77.59% <ø> (ø)
fortran_api	56.15% <ø> (ø)
python_api	80.38% <ø> (ø)
unit_tests	1.24% <ø> (ø)

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[lan496]

I am not satisfied with the generated API docs because they are too long (for example, get_symmetry_dataset). That being said, I will merge this PR if it is acceptable. I think TypedDict for a returned dictionary will improve the readability without losing backward compatibility.

[atztogo]

Thanks @lan496 for a lot of work. I installed autodoc2 using pip and I could generate the documentation using sphinx-autobuild. It looks very good. Originally I never thought spglib python API would be used so much by people. So I made it to be similar to the C functions and the design is not pythonic. Those can be renovated at some point in the future if it will be worth to do it.

[atztogo]

@lan496, any docstring style is fine to me.

[LecrisUT]

Do we use sphinx's python-domain directives instead?

*sphinx-style (it can be used for C/C++ code as well), and yes, I would say it's a better interface and rst/md agnostic. 👍 for that format

[atztogo]

Thanks @lan496, @LecrisUT!