Add Better Docs/Samples for Animations component

We should have the following for the Animations package:

### Tasks
- [ ] Port existing AnimationSet/ImplicitAnimationSet/AnimationBuilder/Connected/ItemsReorderAnimations docs
- [ ] Better introductory guidance on getting started with Animations in general (central 'Animations' page) - how are these things connected, what types of animations (implicit, explicit, connected, etc...)
- [ ] Ensure there's some basic samples on simple animation sets (Explicit) - no extra frills
- [ ] Bring over existing Connected Animations sample
- [ ] Bring over existing ItemsReorderAnimation sample
- [ ] Bring over Scale animation sample

まず Animations パッケージと、AnimationSet、ImplicitAnimationSet、AnimationBuilder、Connected、ItemsReorderAnimations に関する既存のドキュメントおよびサンプルを確認します。中心となる導入を整理し、一覧にあるサンプルを移植して、チェックリストのすべてのタスクが網羅されていることを確認します。