[charts] Add a demo that shows how to add legends to a composed chart

[prakhargupta1]

Related page

https://mui.com/x/react-charts/composition/

Kind of issue

Missing information

Issue description

A user tried to add legends to a composed chart demo through MUI Chat, but failed. This happened probably because there is no demo that showcases this possibility OR maybe even as they mentioned: ChartsLegend component seem to be broken as it doesn't render the legend when used with ChartContainer

Search keywords: legend

[JCQuintas]

There is documentation in two places, one with acknowledgement, and one with example.

We probably need to figure a new place to add this, as it can be a bit tricky.

https://mui.com/x/react-charts/components/#html-components
https://mui.com/x/react-charts/composition/#legend

I've just noticed that we have wrong information on https://mui.com/x/react-charts/composition/#additional-information

We have previously discussed improving/simplifying the DX around this, but it never went forward.

[alexfauquette]

Considered as fix when adding a small demo (line Plot + legend to https://mui.com/x/react-charts/composition/#legend)

[github-actions]

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