User facing changelog for the 7.1 release

Similar to JupyterLab, we should highlight some of the new user-facing features available in Notebook 7.1:

- https://github.com/jupyterlab/jupyterlab/issues/15576
- https://jupyterlab.readthedocs.io/en/latest/getting_started/changelog.html#v4-1