chore: added make test-notebooks command and extended test.yml to run it

[eschmidt42]

This pull request refactors the project's test and notebook workflow for better modularity and clarity, updates dependency management, and reorganizes the notebook directory structure. The most important changes are grouped below:

CI/CD Workflow Improvements

• Split the GitHub Actions workflow into separate jobs for running Python tests and executing notebooks, and updated the installation steps to use new Makefile commands for test and notebook dependencies (.github/workflows/test.yml) [1] [2].

Dependency and Environment Management

• Refactored the pyproject.toml dependency groups to separate test, notebook (nb), and development dependencies, and configured uv to use the test group by default (pyproject.toml) [1] [2].
• Updated the Makefile to add new targets for installing and testing notebooks, improved help messages, and adjusted existing targets to use the new dependency groups (Makefile).

Notebook Organization and Maintenance

• Moved the profiling notebook to nbs/dev/ and made minor code formatting improvements within the notebook (nbs/dev/xgboost-profiling-histogramming-yay-or-nay.ipynb) [1] [2] [3] [4] [5] [6] [7].
• Updated the README to reflect the new notebook directory structure, distinguishing between core and development notebooks (README.md).