Add current versions and supported major versions to Jupyter web site

[JasonWeill]

On the Jupyter install page, 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.

[kumar0205]

Hi maintainers 👋
This seems like a valuable improvement to the Jupyter install page.
My suggestion would be to add:

a table listing each product (JupyterLab, Notebook, etc.),

its latest release, and

the two supported major versions,
along with a short note about maintenance policy.
Please let me know if this direction makes sense — I’m happy to help implement it.