Include examples for `ITransitionGUI` on documentation site

[daenius]

Use Case

The ability to easily create decorators for the various AnimancerTransition<TState> for any situation is one of the best features provided by Animancer and it was demonstrated in the Root Motion examples on the documentation site. Users would benefit from a simple breakdown of how to easily extend the editor UI for their custom subclasses.

Solution

Just something like showing how to add an extra Button and draw an extra Rect to showcase the lifecycle and how each of the functions in ITransitionGUI should be implement would suffice.

Alternatives

Platform Game Kit has this, you could just link the code something like AttackTransition would do the trick too

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Thanks!