Overhaul the top sections of the class reference (Animation classes)

[MewPurPur]

The top sections of classes are often one of the first things users read about them, so they are important to get right.

Part of a series; this is the one dedicated to the animation team. Here are the others so far: #76526 (core), #76600 (animation), #76702 (GUI & text)

In this PR I mostly touched up the short descriptions and occasionally the full descriptions and the tutorials sections. But I also tweaked a little bit of documentation to the side and added a few descriptions.

Rules for brief descriptions

I'm abiding by a set of rules for consistency. I had them in mind when writing all descriptions, but violated them intentionally sometimes.

• Use the articles "a" and "an". This is helpful for words that can be multiple things, i.e. "light" which can be a noun, an adjective, and a verb. This, and the fact it's grammatically correct, helps with reading comprehension in many cases.

• Use a single sentence with punctuation. If two sentences are needed, don't use newline.

• Never use text formatting (i.e. italics) or external links.

• Don't use capitalization for normal words. If referring to a class, use a reference.

• For nodes, always answer "What is this?". For non-node objects, it's also OK to answer "What does this do?"

• Avoid self-referencing, such as "Popup: Popup is a [...]"

• Avoid circular reasoning such as "CanvasLayer: Canvas drawing layer."

  • Exception 1: It's something common folk are familiar with, i.e. "Timer: A countdown timer."
  • Exception 2: The class can't be used by people who don't know the term, i.e. it's okay to say "XML" in XMLParser.

• When a class is abstract, start with "Abstract base class for [...]".

• I often used "Holds X" and "Stores X" to replace "Contains X" - I think it improves clarity that X is the main thing, not just one of the things the resource holds.

• Try to stay under 120 characters without comprimising the quality of writing. (Sometimes it's impossible, in that case, go on for a bit longer)

A long list of brief description changes

Classes not on here are ones I thought to be already excellent, or ones that I didn't realize were "animation". This list might be a slightly outdated (like, by 1%).

Animation: Holds data that can be used to animate anything in the engine.
AnimationLibrary: Stores a set of [Animation] resources.
{Change link title of the tutorial of all the below to "Using AnimationTree"}
AnimationNode: Base class for [AnimationTree] nodes. Not related to scene nodes.
AnimationNodeAnimation: An input animation used in [AnimationNodeBlendTree].
AnimationNodeBlendSpace1D: A set of [AnimationRootNode]s placed on a virtual axis, crossfading between the two adjacent ones. Used by [AnimationTree].
AnimationNodeBlendSpace2D: A set of [AnimationRootNode]s placed on 2D coordinates, crossfading between the three adjacent ones. Used by [AnimationTree].
AnimationNodeBlendTree: A sub-tree of blend type [AnimationNode]s used for complex animations. Used by [AnimationTree].
AnimationNodeOneShot: Plays an animation once in an [AnimationNodeBlendTree].
AnimationNodeOutput: The animation output node of an [AnimationNodeBlendTree].
AnimationNodeStateMachine: A state machine with multiple [AnimationRootNode]s, used by [AnimationTree].
AnimationNodeStateMachinePlayback: Provides playback control for [AnimationNodeStateMachine].
AnimationNodeStateMachineTransition: A transition within an [AnimationNodeStateMachine] connecting two [AnimationRootNode]s.
AnimationNodeSync: Base class for [AnimationNode]s with more than two input ports that must be synchronized.
AnimationNodeTimeScale: A time-seeking animation node used in [AnimationTree].
AnimationNodeTimeSeek: A time-scaling animation node used in [AnimationTree].
AnimationNodeTransition: A transition within an [AnimationTree] connecting two [AnimationNode]s.
AnimationPlayer: A node used for animation playback.
AnimationRootNode: Base class for [AnimationNode]s used by [AnimationTree] to store one or multiple composite animations.
AnimationTree: A node used for advanced animation transitions in an [AnimationPlayer].
Bone2D: A joint used with [Skeleton2D] to control and animate other nodes.
BoneAttachment3D: А node that copies the 3D transform of a bone in its parent [Skeleton3D].
BoneMap: Describes a mapping of bone names for retargeting [Skeleton3D] into common names defined by a [SkeletonProfile].
PhysicalBone2D: A [RigidBody2D]-derived node used to make [Bone2D]s in a [Skeleton2D] react to physics.
PhysicalBone3D: A physics body used to make bones in a [Skeleton3D] react to physics.
Skeleton2D: The parent of a hierarchy of [Bone2D]s, used to create a 2D skeletal animation.
Skeleton3D: A node containing a bone hierarchy, used by [MeshInstance3D] to create a 3D skeletal animation.
SkeletonIK3D: A node used to rotate all bones of a [Skeleton3D] bone chain a way that places the end bone at a desired 3D position.
SkeletonModification2D: Base class for resources that operates on [Bone2D]s in a [Skeleton2D].
SkeletonModification2DTwoBoneIK: A modification that rotates two bones using the law of cosines to reach the target.
SkeletonProfile: Base class for a profile of a virtual skeleton used as a target for retargeting.
Tween: A lightweight object for animating from scripts, assigning each animation to a [Tweener].
Tweener: Abstract base class for all tweeners used by [Tween].

Thanks a ton to Lyuma who helped with and reviewed some of these changes in advance.

[fire]

I was in the conversation with Lyuma and we covered the sticking points of the documentation a week ago?

There's a lot of text to review and I'm a fan of worse is better. Worse is better than perfection.

[MewPurPur]

There are additional changes I haven't talked about (added a few missing descriptions, changed some wording) so it'd be good to take another look anyway.

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.0.4.