Make "This web page was generated from a Jupyter ..." more visible

[hoxbro]

Right now the warning is hidden at the bottom of the web page and could easily be missed.

https://pyviz-dev.github.io/lumen/user_guide/pipeline.html

[philippjfr]

This issue should be moved to nbsite.

[maximlt]

There's a meta-issues on this here: holoviz/holoviz#320

[github-actions]

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