Review the button's onclick window.open for replacement by target=_blank

[Carreau]

Problem

A number of buttons use an onclik window.open to open in a new window, while <a target='_blank'> exist for this purpose.

This is not optimal as a hover on link with target-blank tell you what it does, but not if there is an onclick handler; and create accessibility issues.

Proposed Solution

use <a>s ans style them like the buttons

Additional context

https://github.com/jupyterlite/jupyterlite-sphinx/pull/223/files#r1883631155)