[ci] enable ruff-format on some files, add pre-commit config

[jameslamb]

Contributes to #6304.

• adds auto-formatting of Python code with ruff format
• moves enforcement of that formatting, ruff linter checks, and isort into a pre-commit config
• updates CONTRIBUTING.md with information on how to do that

Notes for Reviewers

The versions of tools I added in .pre-commit-config.yaml are the latest released versions available.

As discussed over in #6304, in this PR I've omitted the directories with a lot of Python code (examples/, python-package/, and tests/). So we can focus on the types of changes that'll be made and just see a few small examples in other code.

Also as described in #6304, I'll add the commit from this PR (and the others that reformat the code) in a .git-blame-ignore-revs file once we're done with this work.

[borchero]

Looks reasonable to me! Thanks for setting this up :)

[jmoralez]

I think having two pyproject.toml files is a bit confusing. If we're going to replace isort with ruff in the future it may be worth to define a ruff.toml and a temporal .isort.cfg instead. WDYT?

[jameslamb]

I think having two pyproject.toml files is a bit confusing. If we're going to replace isort with ruff in the future it may be worth to define a ruff.toml and a temporal .isort.cfg instead. WDYT?

I don't agree that it's confusing.

• project-wide configuration at the top-level pyproject.toml
• details for building the lightgbm Python package in python-package/pyproject.toml

I'd prefer to not have a proliferation of tool-specific config files at the root of the repo. And I like having it at the root of the repo so that if you just call ruff check or isort or mypy or whatever directly from the root of the repo (not using pre-commit), you'll still automatically pick up the configuration used in CI.

However, I feel most strongly about not having the tool-specific config files. So if you don't like having multiple pyproject.toml, would you support adding stuff like this in .pre-commit-config.yaml?

 args: ["--config", "python-package/pyproject.toml"]

And moving the configuration back into there?

[jmoralez]

would you support adding stuff like this in .pre-commit-config.yaml?
args: ["--config", "python-package/pyproject.toml"]
And moving the configuration back into there?

Yes, I think that's clearer

[jameslamb]

Alright no problem, I'm good with that! Just pushed a commit moving the settings back into python-package/pyproject.toml and telling pre-commit to look there for configuration.

[jmoralez]

Thanks!

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