docs: 完善 flexX 转换文档，增加使用场景和示例

[eleliauk]

• Fixed: [Docs]: 改造文档「核心概念 - 转换（Transform）- flexX」 #6677
  目前是用了两个例子 看看可行性如何

Checklist

• npm test passes
• benchmarks are included
• commit message follows commit guidelines
• documents are updated

Description of change

[coveralls]

Pull Request Test Coverage Report for Build 14444491778

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 31 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.02%) to 86.788%

---

Files with Coverage Reduction	New Missed Lines	%
src/transform/utils/helper.ts	1	92.31%
src/runtime/scale.ts	10	93.72%
src/interaction/tooltip.ts	20	91.6%

Totals
Change from base Build 14325874424:	0.02%
Covered Lines:	10745
Relevant Lines:	12000

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💛 - Coveralls

[interstellarmt]

感谢您的贡献 🎉：

有一些的小的建议：

1. 概述和使用场景最好有1个例子能看到效果。
2. 表格里的字符串用行内代码包裹起来，代表是字符串，不用加' '，string这种基础类型定义可以不加行内代码样式

[eleliauk]

例子有什么推荐嘛

[interstellarmt]

例子有什么推荐嘛

像这样有代表性的就行，可以在example里搜一下flexX， https://g2.antv.antgroup.com/examples/general/interval/#bar-flex

[eleliauk]

麻烦再看看呢

[interstellarmt]

例子补充的很不错，示例的代码格式需要修改一下
示例代码为了大模型友好，统一使用 spec 语法。即 chart.options({})，可以在图表示例页面完成切换～
before：

在图表示例里随便找一个例子替换一下代码，然后点击右上角的spec，切换成spec语法
after：