Add draw.io diagrams/charts example to the BookStack demo

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Describe the Scenario

Hello,

I just noticed by chance that BookStack nowadays supports draw.io diagrams/charts. That's cool and I wanted to take a look at it in the demo. Seems like there is no example of this type of content in the BookStack demo. Maybe this can be added to see how it looks like and having an example ready to be edited when you want to try out the demo.

Best Regards,

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