Add XML documentation for Android.Views.Animations

The `Android.Views.Animations` namespace contains **128** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Android.Views.Animations/**' --glob 'ns-Android.Views.Animations.xml' 'to be added' docs\xml
```

Run the provided rg command from the repository root to list the 128 placeholders under docs\xml for Android.Views.Animations, excluding docs\xml\index.xml. Read the matching namespace and API documentation files to understand the surrounding entries and document each placeholder. Done means the namespace no longer contains the reported 'To be added' placeholders.

索引モデルが issue の本文から書いたものです。