Create UObject / Blueprints / C++ API Design Guide

[j9liu]

Over the years we've learned some lessons while developing Blueprints and C++ functions in Cesium for Unreal. We should document these practices to both standardize them and support future contributors.

We have something similar to this in developing-uobjects.md, but this is probably outdated. The scope of this guide could be something like:

• Blueprint API best practices
  • Explaining different metadata / macros for Blueprints
• UObject best practices
  • UX design through UPROPERTY macros (inspired by this guide, but more "do" and "don't" style
• C++ API best practices
  • When to make something a Component vs. Actor
  • Creating Blueprint libraries (see metadata classes for examples)
• Deprecation guide

This is just a list to get started, feel free to add other topics that I've missed! There might be ideas we can borrow from CesiumJS's Coding Guide as well.