copied percentage calculation in example

[thomend]

I added @jonmmease 's implementation of the percentage calculation (#3101) to the diverging stacked bar chart example.

It was mentioned in (#3103) that this would be helpful.

The example now slightly differs from the Vega-Lite example since there is a small error in the hard-coded data provided in the Vega-Lite example (percentage should not be 0 when there are 11 answers):

{
        "question": "Question 2",
        "type": "Strongly agree",
        "value": 11,
        "percentage": 0,
        "percentage_start": 63.6,
        "percentage_end": 63.6
      }

@binste would be great if you can again test run the documentation on your machine since it did not work properly on the github code-space.

[jonmmease]

Thanks a lot @thoemi123!