Sync incremental legend documentation from G2

[Copilot]

Syncs incremental documentation content from G2's legend component docs to ant-design-charts, preserving existing code structure.

Changes

• Added "隐藏图例" section to legend.zh.md: Brings Chinese docs to parity with English version, which already had this section

  // Hide specific channel legend
  ({ legend: { color: false } })
  
  // Hide all legends
  ({ legend: false })

• Updated itemSpacing description: Changed from "配置图例项图标、标签、值之间的间距（启用 focus 时还包括聚焦图标间距）" to "配置图例项图标、标签、值、聚焦图标之间的间距" to match G2's more accurate phrasing

Sync approach

• Synced textual content and descriptions only
• Excluded G2-specific interactive examples (type: 'interval', Chart API) per requirement to preserve ant-design-charts code structure
• Verified remaining sections (layout, size, maxRows/maxCols, etc.) already aligned

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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