Add 1xp docs announcement section to documentation home page

To announce the new documentation experience being released, we should add this announcement section on the documentation home page.

<img width="932" alt="image" src="https://github.com/Qiskit/qiskit_sphinx_theme/assets/59657965/c0606c5a-f94a-43f8-b109-49c975ce457b">

fyi @javabster 