Add animation feature to cargo_features.md (bevyengine#4450)

# Objective

The new feature `animation` is not documented in `cargo_features.md`.

## Solution

Add it there.

Author: infmagic2047

docs/cargo_features.md

@@ -4,6 +4,7 @@
 
 |feature name|description|
 |-|-|
+|animation|Animation support and glTF animation loading.|
 |bevy_audio|Audio support. Support for all audio formats depends on this.|
 |bevy_gilrs|Adds gamepad support.|
 |bevy_gltf|[glTF](https://www.khronos.org/gltf/) support.|