fix(docs): replace broken external image URLs with local assets

- Image lazy demo: use local /avatar/ paths instead of GitHub raw URLs
- Image fallback demo: use local avatar instead of via.placeholder.com
- Card media demo: use local plants.jpg instead of dead pixabay URL

Author: wangdicoder

packages/react/src/card/demo/image.md

@@ -8,7 +8,7 @@ A card using an image to reinforce the content.
 <Card actions={[
     <a key={1} href="">Learn more</a>
   ]}>
-  <Image width="100%" src="https://cdn.pixabay.com/photo/2015/10/10/22/47/koala-981571__480.jpg"/>
+  <Image width="100%" src="/avatar/plants.jpg"/>
   <Card.Content>
     <h2 style={{ margin: 0 }}>Koala</h2>
     <p style={{ marginBottom: 0 }}>The koala is an arboreal herbivorous marsupial native to Australia. It is the only extant representative of the family Phascolarctidae and its closest living relatives are the wombats, which comprise the family Vombatidae. </p>

packages/react/src/image/demo/fallback.md

@@ -5,7 +5,7 @@
 Providing a fallback image for when there is an error loading the src of the image.
 
 ```jsx live
-<Image src="" fallback="https://via.placeholder.com/150"/>
+<Image src="" fallback="/avatar/avatar2.png"/>
 ```
 
 </Demo>

packages/react/src/image/demo/lazy.md

@@ -8,9 +8,9 @@ Set `lazy` and `placehoulder` properties to implement the lazy load.
 <div style={{ height: 250, overflowY: 'scroll' }}>
   <p>Scroll up to make images into the view area.</p>
   <div style={{ height: 350 }}/>
-  <Image lazy round src="https://github.com/wangdicoder/tiny-design/blob/master/site/public/avatar/avatar3.png?raw=true" placeholder="https://github.com/wangdicoder/tiny-design/blob/master/site/public/avatar/avatar3_blur.png?raw=true"/>
+  <Image lazy round src="/avatar/avatar3.png" placeholder="/avatar/avatar3_blur.png"/>
   <div style={{ height: 350 }}/>
-  <Image lazy width={300} height={200} src="https://github.com/wangdicoder/tiny-design/blob/master/site/public/avatar/plants.jpg?raw=true" placeholder="https://github.com/wangdicoder/tiny-design/blob/master/site/public/avatar/plants_blur.png?raw=true"/>
+  <Image lazy width={300} height={200} src="/avatar/plants.jpg" placeholder="/avatar/plants_blur.png"/>
   <div style={{ height: 100 }}/>
 </div>
 ```