docs(option): document missing option properties

[Justin-ZS]

Summary

• document toolbox.backgroundColor in English and Chinese option docs
• expand dataZoom-slider.textStyle docs to include supported text box styles while keeping align, verticalAlign, and rich hidden
• document recursive children for series-tree.data items

Not included

• option.textStyle.backgroundColor: not documented because GlobalTextStyleOption in ECharts source does not support backgroundColor
• dataZoom-slider.dataBackgroundColor: not documented because it is a deprecated legacy option; borderColor is the current documented option
• series-pie.label.textStyle: not documented because textStyle under label is a deprecated compatibility hierarchy that is flattened by the preprocessor; current docs already document the flattened label style properties

Verification

• node build/build-doc.js --env dev
• npm run build:site
• schema check for en/zh option docs confirmed toolbox.backgroundColor, dataZoom-slider text box styles, and series-tree.data.children are generated
• schema check confirmed deprecated dataZoom-slider.dataBackgroundColor, option.textStyle.backgroundColor, and series-pie.label.textStyle are not generated

Fixes apache/echarts#21643