Update extension tutorial to reflect new cookiecutter

[fcollonval]

To reduce maintenance burden this merge https://github.com/jupyterlab/theme-cookiecutter

+1, since the theme cookiecutter can easily get out of sync and doesn't necessarily benefit from the improvements made to the ts cookiecutter here (for example CI and releaser config).

Some follow-up steps that come to mind:

• Archive: https://github.com/jupyterlab/theme-cookiecutter
• Update the extension tutorial
• Update the apod repo

Originally posted by @jtpio in jupyterlab/extension-cookiecutter-ts#196 (comment)