Entity extension script tips page

[RisingThumb]

• Adds the Entity Extension Script field to the reference
• Added a tips page on using it
  • This covers how to generate a choices property of all music from an AudioStreamInteractive resource(a basic usecase with Godot Resources)
  • This also covers it for user-defined resources "Prop" and "AllProps" to make an ent_prop entity. This also includes a small note on using DisplayDescriptor with a class property so it can be seen within Trenchbroom