Move screenshots to assets/

Signed-off-by: Timothy Gu <timothygu99@gmail.com>

index.html

@@ -127,12 +127,16 @@
     Cross compiling
     <a href="http://thebeez.home.xs4all.nl/4tH/">4tH</a>:
     </p>
-    <a href="doc/screenshot-4th-compile.png"><img src="doc/screenshot-4th-compile-small.png" alt="4th-compile"></a>
+    <a href="assets/screenshot-4th-compile.png">
+        <img src="assets/screenshot-4th-compile-small.png" alt="4th-compile">
+    </a>
 
     <p>
     and running it:
     </p>
-    <a href="doc/screenshot-4th-run.png"><img src="doc/screenshot-4th-run-small.png" alt="4th-run"></a>
+    <a href="assets/screenshot-4th-run.png">
+        <img src="assets/screenshot-4th-run-small.png" alt="4th-run">
+    </a>
 </div>
 
 <div class="section">