Document the RESET track in the AnimationPlayer

[and-rad]

Your Godot version:
3.4 & above

Issue description:
The RESET track in the AnimationPlayer node is not documented. Relevant PRs that contain some info about the functionality are:

• Add animation reset track feature godot#43115
• Allow to add RESET values from existing keys godot#55653

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/tutorials/animation/introduction.html talks about creating Animation Players, so this might be a good place to put it.

[zacryol]

should probably be right at the end of this section: https://docs.godotengine.org/en/latest/tutorials/animation/introduction.html#adding-a-track

[zacryol]

should probably be right at the end of this section: https://docs.godotengine.org/en/latest/tutorials/animation/introduction.html#adding-a-track

though there's also https://docs.godotengine.org/en/latest/tutorials/animation/animation_track_types.html