Add current versions and supported major versions to Jupyter web site

On the [Jupyter install page](https://jupyter.org/install), we should indicate the current versions and the supported major versions of our key products. This page should also indicate our policies for maintenance; for example, for JupyterLab and Jupyter Notebook, we only support the two most recently released major versions.

Start at the Jupyter install page and locate its source in this Jekyll site. Review the key products named there, then document their current versions, supported major versions, and maintenance policy, including the two-major-version policy for JupyterLab and Jupyter Notebook. Done means the page clearly presents all requested version and support information.

Written by the indexing model from the issue text.