Revise SDL-0341 - Add Generic HMI Plugin Support

[jacobkeeler]

Revised proposal based on review comments (see https://github.com/smartdevicelink/sdl_evolution/issues/1168#issuecomment-900489117)

Updates

• Use clearer language to differentiate test UIs and integrated HMI components
• Add description of message broker
• Change default entry point of integrated components to use the newly added settings menu in the Generic HMI
• Add details regarding web plugin integration
• Update description of plugin types section
• Rearranged some sections for better flow
• Minor wording updates throughout for clarity

[jacobkeeler]

@theresalech Ready for review

[jacobkeeler]

@theresalech

update description for Sidebar and Menu features This was done via the updates to the plugin configuration details, the "Displaying test plugins in-browser" section, and the "Integrated Plugin Controls" section. In each, I tried to update the terminology to make it clearer what each section covered

consider alternatives to the slide-out menu for a navigation interface This was done here, the image was updated to show the newly added tiled settings menu as the navigation interface, which we implemented instead of the slide-out menu during the latest release. The description was updated according to this change.