chore(tooling): 🔧 replace make with just for cross-platform setup

Replace both Makefiles with justfiles using [unix]/[windows] recipe attributes so just setup works identically on macOS, Linux, and Windows. Rename FEATURES.md to MIGRATION-GUIDE.md to better reflect its purpose.

Review Tasks

• run uvx --from cookiecutter cookiecutter git@gitlab.ethz.ch:geg/templates/python-template.git --checkout chore/make-to-just to create a package (at step 6, select 2 for data_package, which is more comprehensive
• cd into your new package dir
• install just, but don't install direnv (yet!)
• run just setup
• install direnv
• run just setup again