Add XML documentation for Java.Security.Cert

The `Java.Security.Cert` namespace contains **240** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Java.Security.Cert/**' --glob 'ns-Java.Security.Cert.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **291** to **240** (reduced by **51**). Related documentation PRs: [#406](https://github.com/dotnet/android-api-docs/pull/406), [#464](https://github.com/dotnet/android-api-docs/pull/464), [#497](https://github.com/dotnet/android-api-docs/pull/497), [#508](https://github.com/dotnet/android-api-docs/pull/508), [#537](https://github.com/dotnet/android-api-docs/pull/537), [#644](https://github.com/dotnet/android-api-docs/pull/644).

首先对 docs\xml 运行提供的 rg 扫描，然后检查它报告的 Java.Security.Cert/** 文件和 ns-Java.Security.Cert.xml 条目。将 240 个 `To be added` 占位符替换为适当的 XML 文档，并重新运行扫描以确认目标占位符已得到处理。

由索引模型根据 Issue 内容生成。