Add architectural diagram to `devtools_extensions` documentation

Extension authors need to know what type of tools are possible to build at a more granular level. Questions we've received from potential extension authors that highlight the types of questions developers need to answer before deciding to author an extension:
- "Is there a way to submit data to the extension from an external process?"
- "is it possible to load external html files inside the extension?"
- "is it possible to get the current cursor location from the IDE?"
- etc.

An architectural diagram that explains in further detail what is provided by the extension framework and the included connections to the Dart Tooling Daemon and the VM service would make extension capabilities more clear.

首先找到 devtools_extensions 文档，并查看 extension framework 如何连接到 Dart Tooling Daemon 和 VM service。完成的标准是添加一个架构图，向潜在的 extension 作者清楚展示受支持的 extension capabilities 及这些连接。

由索引模型根据 Issue 内容生成。