This issue will be used for the following application framework documentation improvements:

• Move "Application Framework" section from "Developer Guides" to "AGL Components"
• Potentially move / replace "APIs and Services" under "AGL Components"
• Rework application framework high-level description to explain preference for gRPC usage instead of D-Bus
• Update applaunchd documentation to
  • remove D-Bus API mentions
  • remove agl-session user session mentions
  • document sandboxing example configuration and discuss other potential configurations
• Add documentation of the radio service and its gRPC API.
• Add initial documentation of the HVAC and mixer services and their VSS signal API.