Adding docs example of using styleKey (#243)

* Added example of using styleKey

* Added one more indent fix

---------

Co-authored-by: Louie Bertoncin 🥏 <loberton@microsoft.com>

Author: louiebert

USAGE.md

@@ -152,7 +152,7 @@ Note that only react-native-xaml components will respect the `gridRow`/`gridColu
 ## NavigationView
 
 ```jsx
-  const [text, setText] = React.useState('initial text');
+const [text, setText] = React.useState('initial text');
 
 <WinUI.NavigationView width={700} height={700}>
   <WinUI.NavigationViewItem content='Item 1' onTapped={() => setText('text #1')} priority={NavigationViewPriority.MenuItem}>
@@ -224,7 +224,7 @@ const [showState, setShowState] = useState(ContentDialogState.Hidden);
 ## SplitView
 
 ```jsx
-  const [isOpen, setIsOpen] = useState(false);
+const [isOpen, setIsOpen] = useState(false);
 // ...
 <SplitView
   isPaneOpen={isOpen}
@@ -249,13 +249,13 @@ const [showState, setShowState] = useState(ContentDialogState.Hidden);
 </SplitView>
 
 
-  <Button
-    foreground="#992222"
-    onTapped={a => {
-      setIsOpen(!isOpen);
-    }}
-    content="button"
-  />
+<Button
+  foreground="#992222"
+  onTapped={a => {
+    setIsOpen(!isOpen);
+  }}
+  content="button"
+/>
 ```
 
 ## Path & vector graphics
@@ -273,12 +273,25 @@ const [showState, setShowState] = useState(ContentDialogState.Hidden);
 
 ## Lightweight styling
 
+Using `resources` to style specific aspects of a control
 ```jsx
-<Button content="Hello with style!" resources={{ 
+<Button 
+  content="Hello with style!" 
+  resources={{ 
     ButtonForeground: "#00fff1",
     ButtonForegroundPressed: "#2090ff",
-    }} />
+  }} 
+/>
+```
+
+Alternatively, use `styleKey` to leverage system theme styles. 
+```jsx
+<Button 
+  content="Hello with accent style!" 
+  styleKey="AccentButtonStyle"
+/>
 ```
+> Note: `"AccentButtonStyle"` used in this example [is defined here](https://github.com/microsoft/microsoft-ui-xaml/blob/9052972906c8a0a1b6cb5d5c61b27d6d27cd7f11/dev/CommonStyles/Button_themeresources.xaml#L239). Controls will have their supported `styleKey`s defined in associated *_themeresources.xaml files like the one linked.
 
 ## Event args