docs(sigs): add SIG overview and participation guide

[openjiuwen-sync-bot]

Paired: GitHub #7 ↔ GitCode !88

What type of PR is this?

/kind documentation

What does this PR do?

Fix #I51: 重写 sigs/README.md，为 SIG 目录页补充浏览与参与导航，降低新贡献者参与门槛。

主要变更：

1. SIG 总览表：列出当前全部 16 个 SIG 的名称、描述、成熟度等级、Maintainer 与负责仓库
2. 参与指引：说明如何加入一个已有的 SIG（签署 CLA、联系 Maintainer、参与例会、认领任务）
3. 新建 SIG 流程：参考社区章程提供提案要素与 TC 审批流程
4. SIG 成熟度等级说明：startup / active / retired

Self-checklist：

• • 文档：涉及 docs 修改，已提交至 community 仓库
• • 提交信息符合 Conventional Commits 规范并包含 Signed-off-by

验证情况：

• 在本地渲染 markdown，表格与相对链接（sig-info.yaml、社区章程）均可正常跳转

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