Add architectural diagram to devtools_extensions documentation

[kenzieschmoll]

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.