Texture Atlas migration should be complete

## Problem

The [migration regarding `TextureAtlas`](https://bevyengine.org/learn/migration-guides/0-14-to-0-15/#move-textureatlas-into-uiimage-and-remove-impl-component-for-textureatlas) tells the user to use an `UiImage`.

However a [later section](https://bevyengine.org/learn/migration-guides/0-14-to-0-15/#uiimage-imagenode-uiimagesize-imagenodesize) reveals that `UiImage` has been replaced and should be changed to `ImageNode`

## Solution

The section for `TextureAtlas` should either explain the full migration, or at least point directly to the next step.

Open the 0.14-to-0.15 migration guide and compare the TextureAtlas section with the later UiImage/ImageNode section. Update the TextureAtlas migration text to include the full migration or link directly to the next step. Done when readers are not directed to the obsolete UiImage API.

Written by the indexing model from the issue text.