Cookiecutter update, add AI policy

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR closes [pre-commit.ci] pre-commit autoupdate #101 (redundant)
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• CHANGELOG.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Updates the cookiecutter template to latest

Does this PR introduce a breaking change?

Yes. There are a handful of new systems here:

• prek has replaced pre-commit. This tool is a drop-in replacement built in Rust that uses the same configuration files. Documentation has been adjusted to support it.
• The package now uses dependency-groups to manage non-user dependencies used for documentation and development. Requires pip >=25.2.
• tox now uses the TOML spec for its configuration. It also relies more heavily on dependency-groups and Makefile recipes.
• The Makefile now contains logic to handle dependency installation.
• Linting checks are much more elaborate/consistent.

In addition, the project now contains a base template for generative AI code contribution disclosures. The default "standard" template has been added. See AGENTS.md, AI_POLICY.md and CONTRIBUTING.rst for more information. Other settings are possible (strict and No AI policy).

Other information:

A new workflow has also been added that automatically accepts "safe" Dependabot updates. This has been employed in other Hydrologie projects as well.