feat(template): ✨ support conditionally generating pure data analysis projects

- Revised `cookiecutter.json` to prompt for `standard` or `data_project` repository types.
- Established a unified scientific folder structure containing `data/`, `outputs/`, `scripts/`, and `tests/` with `.gitkeep` placeholders for both templates.
- Added Jupyter Notebook safety ignores (`*.ipynb` and `.ipynb_checkpoints/`) to `.gitignore` to keep accidental notebooks out of version control.
- Conditioned `pyproject.toml` to generate an empty dependency block for `standard` projects, or an instructional commented-out GEG GitLab dependency block for `data_project`s.
- Updated template `README.md` to dynamically document the generated layout and explicitly note the data-pipeline dependency requirement for `data_project`s.
- Removed auto-deletion script from `hooks/post_gen_project.py` to ensure `src/` and `tests/` directories are retained unconditionally.