notebook: Use a box layout

And incidentally make the CSS node docs correct again.
8 jobs for master in 15 minutes and 2 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #431568
fedora-x86_64

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passed #431569
win32
msys2-mingw32

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  Flatpak
manual #431570
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flatpak-manual:demo
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flatpak-manual:icon-browser
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flatpak-manual:widget-factory
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flatpak-master:demo

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flatpak-master:icon-browser

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flatpak-master:widget-factory

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