Please add a cell toolbar example

[MRYingLEE]

There is a well-known example, https://github.com/jupyterlab-contrib/jlab-enhanced-cell-toolbar. It is cool, but it seems too complicated.
Can you provide a simple example, which may use '@jupyterlab/cell-toolbar'?

1. Some buttons may be available for Code cell only.
2. Some buttons may be available for Markdown cell only.
3. Some buttons may be enabled for some kind of content.

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[jtpio]

Thanks @MRYingLEE for the suggestion 👍

Yes having such example would be great.

[jtpio]

cc @brichet who I think was looking into this recently.

[brichet]

Thank you for the reminder @jtpio 😄

[brichet]

@jtpio, any opinion about adding a new example or extending to the current toolbar-button example ?

[jtpio]

Maybe a new one for simplicity, so the example focuses on extending the cell toolbar only? It might also be easier to find from the top-level folder or readme.