feat: new 500.astro error prop (#8392)

Co-authored-by: Emanuele Stoppa <my.burning@gmail.com>
Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

src/content/docs/en/basics/astro-pages.mdx

@@ -110,6 +110,28 @@ If an error occurs rendering this page, your host's default 500 error page will
 
 During development, if you have a `500.astro`, the error thrown at runtime is logged in your terminal, as opposed to being shown in the error overlay.
 
+### `error`
+
+<p><Since v="4.11.0" /></p>
+
+`src/pages/500.astro` is a special page that is automatically passed an `error` prop for any error thrown during rendering. This allows you to use the details of an error (e.g. from a page, from middleware, etc.) to display information to your visitor.
+
+The error prop's data type can be anything, which may affect how you type or use the value in your code:
+
+```astro title="src/pages/500.astro"
+---
+interface Props {
+    error: unknown
+}
+
+const { error } = Astro.props
+---
+
+<div>{error instanceof Error ? error.message : 'Unknown error'}</div>
+```
+
+To avoid leaking sensitive information when displaying content from the `error` prop, consider evaluating the error first, and returning appropriate content based on the error thrown. For example, you should avoid displaying the error's stack as it contains information about how your code is structured on the server
+
 ## Page Partials
 
 <p><Since v="3.4.0" /></p>