Create manual explaining the usage of Skeleton3D, SkeletonModifier3D, BoneAttachment3D and subclasses under animations section

**Your Godot version:**
v4.7.stable

**Issue description:**
Almost all 3D games use skeletons to animate meshes. They are, generally, one of the most important fundamental features for creating good 3D games. Skeletons are authored by artists who are generally averse to reading through the class docs. There are some explanations scattered around the docs in various places explaining tidbits of how to use skeletons for animation and how to modify them, but it can be difficult to find relevant information when working with skeletons. 3D artists creating assets for Godot or working in Godot would benefit greatly from having a manual on the common features and uses for skeletons and skeleton modifiers in Godot that is easy to find and gives basic explanations of the most important things they need to know.

Specifically, sections explaining how to achieve common effects such as:
- Animated hitboxes using BoneAttachment3D
- Secondary animation (jiggle physics) on hair and loose fabric using SpringBoneSimulator3D
- Using TwoBoneIK3D to snap a character's feet to floors with bumpy surfaces
- Using LookAtModifier3D to make NPCs look at the player

These are all trivial things to implement with little or no code if you know what buttons to press and are very common features so I think it's worth pointing underexperienced users in the right direction as well as giving programmers an easy page to link to when working with artists.

It would also be nice to have more complex guides for programmers such as:
- Animating procedural geometry using generated skeletons and bones

As well as linking to or maybe even moving:
- "Retargeting 3D Skeletons" section in "Assets Pipeline" manual
- "Ragdoll System" section in "Physics" manual

Start by reviewing the animations section and the existing “Retargeting 3D Skeletons” and “Ragdoll System” sections in the Assets Pipeline and Physics manuals. Scope the manual around Skeleton3D, SkeletonModifier3D, BoneAttachment3D, SpringBoneSimulator3D, TwoBoneIK3D, and LookAtModifier3D, including the listed common effects. Done means artists can find practical guidance and programmers can link to it, with the related sections linked or moved as appropriate.

Written by the indexing model from the issue text.