Update AnimationTree documentation for `SyncMode::CYCLIC` addition in BlendSpace1D and BlendSpace2D

[Calinou]

Your Godot version: 4.7.dev4

Issue description: The AnimationTree documentation should be updated for godotengine/godot#117275, which is now merged. The feature is quite complex, so it deserves its own section in the page.

URL to the documentation page (if already existing): https://docs.godotengine.org/en/latest/tutorials/animation/animation_tree.html

@uno1982 Could you look into this if possible? If not, let me know and someone else can tackle this.