[Sitemaps.md] Refactor Charts section

[Confectrician]

Ported from #959.

We should:

• Completely rework the charts section
• Add up to date info about persistence
• Add some example image

Original TODO comment(s) in sitemaps.md:

<!-- TODO: Is this statement correct?  Isn't there a default persistence setting in paperUI? -->

<!-- TODO: This paragraph needs an update -->

<!-- TODO
![Presentation of the Chart element in BasicUI](images/sitemap_demo_chart.png)
-->

[blilleike]

I think, the beginner label is misleading.
I have not mastered persistence and charts yet, so I do not see how I could start with this issue.
Or I misunderstood the task or the beginner label

[Confectrician]

Hey @blilleike,

I have used the beginner label, because i think this issue could be a good one to start.
It wasn't meant as "i can do that on my own very quickly" here, so you are right and it could be a bit misleading.

We can of course have a look on that section together and check if we can precise the task a bit. :)