Fix outputs DOM structure

[martinRenou]

References

Another step for resolving https://github.com/voila-dashboards/voici/issues/70. It does not look exactly the same as classic Voila still, but it's already better.

Code changes

We were missing the collapser and the SimplifiedAreaOutput was missing some DOM elements.

[github-actions[bot]]

:point_left: Try it on ReadTheDocs

[martinRenou]

Thanks!