docs: cap height of artificially-slow example output (#8486)

Author: lee-eojin

src/content/reference/react/useDeferredValue.md

@@ -705,6 +705,8 @@ export default SlowList;
 
 ```css
 .items {
+  max-height: 1200px;
+  overflow-y: auto;
   padding: 0;
 }
 
@@ -782,6 +784,8 @@ export default SlowList;
 
 ```css
 .items {
+  max-height: 1200px;
+  overflow-y: auto;
   padding: 0;
 }
 

src/content/reference/react/useTransition.md

@@ -733,6 +733,11 @@ export default function ContactTab() {
 ```
 
 ```css
+.items {
+  max-height: 1200px;
+  overflow-y: auto;
+}
+
 button { margin-right: 10px }
 b { display: inline-block; margin-right: 10px; }
 .pending { color: #777; }
@@ -888,6 +893,11 @@ export default function ContactTab() {
 ```
 
 ```css
+.items {
+  max-height: 1200px;
+  overflow-y: auto;
+}
+
 button { margin-right: 10px }
 b { display: inline-block; margin-right: 10px; }
 .pending { color: #777; }
@@ -1046,6 +1056,11 @@ async function getPosts() {
 ```
 
 ```css
+.items {
+  max-height: 1200px;
+  overflow-y: auto;
+}
+
 button { margin-right: 10px }
 b { display: inline-block; margin-right: 10px; }
 .pending { color: #777; }
@@ -1210,6 +1225,11 @@ async function getPosts() {
 ```
 
 ```css
+.items {
+  max-height: 1200px;
+  overflow-y: auto;
+}
+
 button { margin-right: 10px }
 b { display: inline-block; margin-right: 10px; }
 .pending { color: #777; }