Migration guide for Unity users

[willeastcott]

Lots of Unity users are turning to PlayCanvas to get good WebGL support/performance. Therefore, I think it would be beneficial to add a Migration Guide to the User Guide to explain how to handle things like:

• Pre-fabs
• UI creation
• Managing what is rendered by what camera
• ...and whatever else makes sense

[yaustar]

From feedback on Discord by TotalGeezer:

• Unity monobehaviours are components whereas PlayCanvas ScriptTypes are under the script component
• Components are directly access under the Entity, no need for GetComponent
• Only one instance of each component and ScriptType per entity
• No class definitions for ScriptTypes, 'declare' member variables in initialize being the common pattern (note on JS being a dynamic language)
• Note on keyboard shortcuts of QWER (select, translate, rotate, scale) being mapped to 123 in PlayCanvas
• Debugging is done in the Browser Devtools
• Debug drawcalls with Spector.js
• Profiler is available in Alt + P or the Browser Devtools

[yaustar]

Unreal has done a guide here that we could use as a template: https://docs.unrealengine.com/5.0/en-US/unreal-engine-for-unity-developers/?s=03