doc: update image style

docs/static/pywebio.css

@@ -20,4 +20,10 @@
 /* Table cell */
 td .line-block {
     margin-bottom: 0 !important;
+}
+
+.rst-content img {
+    box-shadow: 0px 4px 20px 0px rgba(0,0,0,.35);
+    -moz-box-shadow: 0px 4px 20px 0px rgba(0,0,0,.35);
+    -webkit-box-shadow: 0px 4px 20px 0px rgba(0,0,0,.35);
 }