DOC: add extensions in extensions.tpl

[rootsmusic]

In extensions.tpl, I suggest adding itables, QgridNext, and ipyaggrid.

[fcollonval]

Thanks for the suggestions, feel free to open a PR - I would add them in the Viewers / Renderers section.

You can use that link to start editing the file in your browser: https://github.com/jupyterlab-contrib/jupyterlab-contrib.github.io/edit/main/extensions.tpl