Fix broken IPython Reference URL in help_links

[goelakash]

The current https://ipython.org/documentation.html returns a 404 which no longer exists. The correct IPython documentation home is now https://ipython.readthedocs.io/en/stable/.

This URL is surfaced in the Jupyter Notebook / JupyterLab Help menu (under "IPython Reference") via help_links, so the broken link is user-visible.

$ curl -sI https://ipython.org/documentation.html | head -1
HTTP/2 404

$ curl -sI https://ipython.readthedocs.io/en/stable/ | head -1
HTTP/2 200