Fix shifted joint radius index in SpringBone gizmo and remove unused end_bone_tip_radius property

[TokageItLab]

• Follow up Implement SpringBoneSimulator3D to wiggle chained bones #101409

The joint drawing index was misaligned and will be corrected.

Before this fix, the root bone's radius was being treated as if it were invalid, when in fact it was valid.

Wrong gizmo:

-(1)-(2)-(3)-(end)

Wrong gizmo (extend end bone):

-(1)-(2)-(3)-(end)-(tip)

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Internally, it behaved like this in the colliding process:

-(root)-(1)-(2)-(3)

Internally, it behaved like this in the colliding process (extend end bone):

-(root)-(1)-(2)-(3)-(end)

So this PR fix gizmo index to the latter.

The tip radius was only being used for drawing purposes, but was not actually working in colliding process. Since the end bone's radius is actually used and tip radius is not doing anything, remove it.

Also write end_bone behavior in detail in the documentation.

[akien-mga]

Thanks!