Avoid a re-build of the entire book with GitHub actions

[fmaussion]

Hi! We are build our JupyterBook with GitHub actions - this works very nicely!

Unless I'm mistaken, the entire book seems to be rebuilt at each PR / Merge if we follow these instructions. Is there a way to obtain the previous build from the gh-pages branch and avoid a rebuild of the notebooks that haven't changed? If yes it would be great! It would be worth documenting on the docs as well (I could volunteer for that).

Thanks!