Docs: Add pie chart docs (#31926)

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* Update panel-legend.md

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docs/sources/shared/visualizations/legend-mode.md

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+---
+title: Legend mode and legend placement
+---
+
+### Legend mode
+
+Use these settings to refine how the legend appears in your visualization.
+
+- **List -** Displays the legend as a list. This is a default display mode of the legend.
+- **Table -** Displays the legend as a table.
+- **Hidden -** Hides the legend.
+
+### Legend placement
+
+Choose where to display the legend.
+
+- **Bottom -** Below the graph.
+- **Right -** To the right of the graph.