Docs: move common things for burndown and burnup charts to a common section

What does this MR do?

As suggested by @psimyn the shared information about the total number of issues respectively the total weight for burndown/burnup charts was moved to a shared space at the top. I also explained the working of burnup charts in a more detailed manner, using the words from #268034.

On the index page, I added the burnup chart and changed the image accordingly.

Related issues

See #268034

Author's checklist

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