Flax for Unity developers: Add info about Flax's equivalent to ScriptableObject

I think it would be useful to mention that Flax's custom assets are effectively analogous to Unity's ScriptableObjects.
Currently, it's not that easy to discover that Flax has this capability, primarily due to the large difference in terminology.

At the least, it could be added to the Terminology table, showing "ScriptableObject" and "Custom Asset Type"
https://docs.flaxengine.com/manual/get-started/flax-for-unity-devs/index.html#terminology
With "Custom Asset Type" linking to https://docs.flaxengine.com/manual/scripting/tutorials/custom-asset.html

There could also be a "ScriptableObject vs Custom Asset Type", section like the existing ones for GameObject and MonoBehavior.

There's already an Assets section
https://docs.flaxengine.com/manual/get-started/flax-for-unity-devs/index.html#assets
I'm not sure if this should also mention ScriptableObject, or if that would be a bit redundant.