[Docs] Dropdown cpp code examples

[mythrz]

• Dropdown cpp code examples
• Create Component mentions how to convert a game component to ui component
• Working with UI components mentions how to edit cmakelists to set as build dependcy "Lyshine"

Change summary

Revisiting the UI documentation for LyShine. Part of #2705
Noticed a few things:

• there was no mention on how to convert a "Game" gem to a "UI" gem
• there was no mention on how to edit the CMakeLists.txt to have LyShine as a build dependency
• there was no mention on how to include and inherit the bus
• there was no mention on how to override the functions/methods/events

Submission Checklist:

• Descriptive active voice - Do descriptive sentences have a clear subject and action verb?
• Answer the question at hand - Does the documentation answer a what, why, how, or where type of question?
• Consistency - Does the content consistently follow the Style Guide?
• Help the user - Does the documentation show the user something meaningful?