chore: Migrate to pyproject.toml

[tedil]

In this PR:

• the old setuptools setup.py is replaced by a pyproject.toml (e.g. for use with uv).
• package layout is migrated to the src/ package layout
• github workflows for linting and testing are updated
• linting and formatting is now done via ruff (instead of flake + isort + black)
• readthedocs setup has been adjusted for v2

[github-actions]

Please check your code via make lint.

[tedil]

Right now, 3 2 tests are failing:

1. test_archive_readme.py::test_run_archive_readme_smoke_test: this is because the symlinks in the test directory don't point anywhere meaningful, i.e. the files simply do not exist within this repository
2. test_seasnap_itransfer_results.py::test_run_seasnap_itransfer_results_smoke_test: this fails because there's some wonky logic for determining the source files from the input string (cmd_block) and subsequent issues with test file paths. it instantiated its own mock filesystem instead of using the existing fixture
3. test_snappy_workflows.py::test_could_import_module tests whether snappy_pipeline can be imported, which is not listed as a dependency anywhere.

---

1. can probably be fixed by ensuring these files actually exist.
2. fixed
3. it should be mentioned somewhere that some commands are only available when there's a snappy_pipeline installation within the same environment?

[Nicolai-vKuegelgen]

LGTM