fix: Breaking up a sentence to make it easier to understand

The line `Trees are a relationship model between items and UI is often represented using tree structures` was a *slightly* confusing to me. I twice read it as `Trees are a relationship model between items and UI` which was incorrect. Therefore, I think it would be better to break the sentence into two which would convey make it easier to read and convey the correct meaning. My apologies if this goes against the convention used in Meta docs.

Author: moijes12

src/content/learn/understanding-your-ui-as-a-tree.md

@@ -20,7 +20,7 @@ React, and many other UI libraries, model UI as a tree. Thinking of your app as
 
 ## Your UI as a tree {/*your-ui-as-a-tree*/}
 
-Trees are a relationship model between items and UI is often represented using tree structures. For example, browsers use tree structures to model HTML ([DOM](https://developer.mozilla.org/docs/Web/API/Document_Object_Model/Introduction)) and CSS ([CSSOM](https://developer.mozilla.org/docs/Web/API/CSS_Object_Model)). Mobile platforms also use trees to represent their view hierarchy.
+Trees are a relationship model between items. The UI is often represented using tree structures. For example, browsers use tree structures to model HTML ([DOM](https://developer.mozilla.org/docs/Web/API/Document_Object_Model/Introduction)) and CSS ([CSSOM](https://developer.mozilla.org/docs/Web/API/CSS_Object_Model)). Mobile platforms also use trees to represent their view hierarchy.
 
 <Diagram name="preserving_state_dom_tree" height={193} width={864} alt="Diagram with three sections arranged horizontally. In the first section, there are three rectangles stacked vertically, with labels 'Component A', 'Component B', and 'Component C'. Transitioning to the next pane is an arrow with the React logo on top labeled 'React'. The middle section contains a tree of components, with the root labeled 'A' and two children labeled 'B' and 'C'. The next section is again transitioned using an arrow with the React logo on top labeled 'React DOM'. The third and final section is a wireframe of a browser, containing a tree of 8 nodes, which has only a subset highlighted (indicating the subtree from the middle section).">