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

Run uvx --from cookiecutter cookiecutter git@gitlab.ethz.ch:geg/templates/package-template.git -c feat/mult-repo-types to test this MR.

• Added repo_type prompt to cookiecutter.json to allow users to choose between a full "package" or a pure research "project".
• Added standard data science directories (data/, outputs/, scripts/) to the blueprint with .gitkeep placeholders.
• Updated post_gen_project.py hook to dynamically delete the src/ and tests/ directories if the "project" type is selected.
• Updated template README.md structure block with Jinja tags to dynamically describe the generated layout.
• Added Jupyter Notebook safety ignores (*.ipynb and .ipynb_checkpoints/) to .gitignore to keep accidental notebooks out of version control.
• includes all changes in !9 (merged), which will be automatically merged if this merged first