Use pyproject toml and update documentation

[morcuended]

• Use only pyproject.toml instead of setup.cfg and setup.py as suggested in project-template-python-pure
• Moved code inside src/
• Update the home page of documentation and use PyData theme
• Linting using Ruff instead of pyflakes
• Fix some linting complaints
• Run tests using Python 3.9, 3.10 and 3.11
• Use just conda versions of ctapipe_io_lst and lstchain in environment.yml

It has no side effects on the way it was working before.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (70e7b03) 81.47% compared to head (dd6267a) 81.54%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #252      +/-   ##
==========================================
+ Coverage   81.47%   81.54%   +0.06%     
==========================================
  Files          57       57              
  Lines        4788     4789       +1     
==========================================
+ Hits         3901     3905       +4     
+ Misses        887      884       -3     

Files	Coverage Δ
src/osa/__init__.py	100.00% <ø> (ø)
src/osa/_dev_version/__init__.py	100.00% <ø> (ø)
src/osa/configs/config.py	88.88% <ø> (ø)
src/osa/configs/datamodel.py	100.00% <ø> (ø)
src/osa/configs/options.py	100.00% <ø> (ø)
src/osa/conftest.py	92.67% <ø> (ø)
src/osa/high_level/significance.py	52.58% <ø> (ø)
src/osa/high_level/tests/test_significance.py	100.00% <ø> (ø)
src/osa/job.py	79.63% <ø> (ø)
src/osa/nightsummary/database.py	42.18% <ø> (ø)
... and 47 more

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[morcuended]

@marialainez please review. Even if it seems that there are many changes, it is just moving the files inside src, minor cosmetics stuff here and there, docs update, start using pyproject.toml for creating the python lstosa package and some updates in the CI (after some trials and errors) to test several python versions and update some versions of GitHub actions that we use.

Important: I realized that we need to stick with conda/mamba env creation for proper handling of dependencies at least in the CI and it is recommended also elsewhere.

The usage of lstosa remains the same.