[charts] Add radial bar overview demo with custom overlays

[JCQuintas]

Summary

Adds an overview demo to the Radial Bars docs page, showcasing the RadialBarChart with custom SVG overlays.

The demo visualizes average trust in others across Europe (Eurostat ilc_pw03, population aged 16+) between 2013 and 2025, inspired by The European Correspondent's infographic:

• Blue bars: 2025 trust (real chart series, keeps tooltips/highlight)
• Orange arc line: 2013 reference level, drawn per country
• Trend markers: up/down triangle offset from the relevant bar by a static distance
• Dashed EU average ring (Eurostat EU27 2025 figure)
• Custom item tooltip showing 2025, 2013, and the change

Details

• Overlays are passed as children and read the polar scales through the public useRotationAxis / useRadiusAxis / useDrawingArea hooks (shared usePolarGeometry helper) — no internals.
• New dataset dataset/europeanYouthTrust.ts.
• Overview section moved to the top of the page; copy made capability-focused.

[code-infra-dashboard]

Deploy preview

• docs/data/charts/radial-bars/radial-bars.md

Bundle size

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@mui/x-data-grid	0B(0.00%)	0B(0.00%)
@mui/x-data-grid-pro	0B(0.00%)	0B(0.00%)
@mui/x-data-grid-premium	0B(0.00%)	0B(0.00%)
@mui/x-charts	0B(0.00%)	0B(0.00%)
@mui/x-charts-pro	0B(0.00%)	0B(0.00%)
@mui/x-charts-premium	0B(0.00%)	0B(0.00%)
@mui/x-date-pickers	0B(0.00%)	0B(0.00%)
@mui/x-date-pickers-pro	0B(0.00%)	0B(0.00%)
@mui/x-tree-view	0B(0.00%)	0B(0.00%)
@mui/x-tree-view-pro	0B(0.00%)	0B(0.00%)
@mui/x-license	0B(0.00%)	0B(0.00%)

Details of bundle changes

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