fix(docs): add example for pageContext usage within a page (#11580)

## Description

This page explains very well that you can pass `context` to a page automatically created, but it's missing an example of how to use it in a page via `pageContext`. I think it is important to explicitly explain it because the name of the variable is different to the prop name.
I added an example.

## Related Issues
I didn't find any related issues.

docs/docs/creating-and-modifying-pages.md

@@ -152,3 +152,15 @@ exports.onCreatePage = ({ page, actions }) => {
   })
 }
 ```
+
+On your pages and templates, you can access your context via the prop `pageContext` like this:
+
+```jsx
+import React from "react"
+
+const Page = ({ pageContext }) => {
+  return <div>{pageContext.house}</div>
+}
+
+export default Page
+```