Add docs about SSR caching (yes, you can do this with Next.js) (#36588)

I've heard a lot of folks not realize this is possible 😄 Hopefully this helps.

Author: leerob

docs/basic-features/data-fetching/get-server-side-props.md

@@ -74,6 +74,32 @@ export async function getServerSideProps() {
 export default Page
 ```
 
+## Caching with Server-Side Rendering (SSR)
+
+You can use caching headers (`Cache-Control`) inside `getServerSideProps` to cache dynamic responses. For example, using [`stale-while-revalidate`](https://web.dev/stale-while-revalidate/).
+
+```jsx
+// This value is considered fresh for ten seconds (s-maxage=10).
+// If a request is repeated within the next 10 seconds, the previously
+// cached value will still be fresh. If the request is repeated before 59 seconds,
+// the cached value will be stale but still render (stale-while-revalidate=59).
+//
+// In the background, a revalidation request will be made to populate the cache
+// with a fresh value. If you refresh the page, you will see the new value.
+export async function getServerSideProps({ req, res }) {
+  res.setHeader(
+    'Cache-Control',
+    'public, s-maxage=10, stale-while-revalidate=59'
+  )
+
+  return {
+    props: {},
+  }
+}
+```
+
+Learn more about [caching](/docs/going-to-production.md).
+
 ## Does getServerSideProps render an error page
 
 If an error is thrown inside `getServerSideProps`, it will show the `pages/500.js` file. Check out the documentation for [500 page](/docs/advanced-features/custom-error-page#500-page) to learn more on how to create it. During development this file will not be used and the dev overlay will be shown instead.