Sync legend configuration properties from G2 v5 to ant-design-charts

[Copilot]

Synchronizes incremental documentation updates from G2 v5 legend docs to ant-design-charts legend documentation, adding 7 new configuration properties and updating 3 existing descriptions.

New Properties

• length - Controls legend container length on main axis (width for horizontal, height for vertical)
• itemSpan - Space allocation array for marker, label, and value components [1, 1, 1]
• poptip - Tooltip configuration for truncated legend items
• focus - Enables focus icon for highlighting specific data series
• focusMarkerSize - Focus icon size (default: 12)
• defaultSelect - Pre-select legend items on initial render
• render - Custom HTML renderer for legend content

Updated Descriptions

• size - Clarified as legend container size (cross-axis dimension)
• itemSpacing - Updated from [8, 8] to [8, 8, 4] (fourth value applies when focus enabled)
• nav - Added vertical layout pagination tip with configuration example

Example Usage

legend: {
  color: {
    focus: true,
    focusMarkerSize: 14,
    defaultSelect: ['Series A', 'Series B'],
    poptip: {
      render: (item) => `Full name: ${item.label}`,
      position: 'top'
    }
  }
}

All code examples use Ant Design Charts syntax without G2-specific fields like type: 'interval' or type: 'view'.

Original prompt

将 http://github.com/antvis/g2/blob/v5/site/docs/manual/component/legend.*.md 目录下的文档内容同步到 https://github.com/ant-design/ant-design-charts/blob/master/site/docs/options/plots/legend.*.md 文件中。

同步要求：

1. 不能全量覆盖，同步增量内容即可
2. 示例代码仅同步配置（options），代码结构不要改动

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