feat: middleware and virtual routes

src/content/docs/en/guides/middleware.mdx

@@ -5,17 +5,9 @@ i18nReady: true
 ---
 import PackageManagerTabs from '~/components/tabs/PackageManagerTabs.astro';
 
-**Middleware** allows you to intercept requests and responses and inject behaviors dynamically every time a page or endpoint is about to be rendered. 
+**Middleware** allows you to intercept requests and responses and inject behaviors dynamically every time a page or endpoint is about to be rendered. This rendering occurs at build time for all prerendered pages, but occurs when the route is requested for pages rendered on demand.
 
-This also allows you to set and share request-specific information across endpoints and pages by mutating a `locals` object that is available in all Astro components and API endpoints
-
-Middleware is available in both SSG and SSR Astro projects.
-
-:::note
-On all prerendered pages, the middleware runs only during the **build process** to create static pages and does not run in the deployed website. This applies to all pages in a `static` (SSG) project. This also includes prerendered pages in `hybrid` mode (default) and any pages in `server` mode with `export const prerender = true`.
-
-For on-demand rendered pages in `server` (default) or `hybrid` (with `export const prerender = false`) mode, the middleware runs in the deployed website when the route is requested.
-:::
+Middleware also allows you to set and share request-specific information across endpoints and pages by mutating a `locals` object that is available in all Astro components and API endpoints. This object is available even when this middleware runs at build time.
 
 ## Basic Usage
 
@@ -200,3 +192,9 @@ greeting response
 auth response
 validation response
 ```
+
+## Error pages
+
+Middleware will attempt to run for all on-demand rendered pages, even when a matching route cannot be found. This includes Astro's default (blank) 404 page and any custom 404 pages. However, it is up to the [adapter](/en/guides/server-side-rendering/) to decide whether that code runs. Some adapters may serve a platform-specific error page instead.
+
+Middleware will also attempt to run before serving a 500 error page, including a custom 500 page, unless the server error occured in the execution of the middleware itself. If your middleware does not run successfully, then you will not have access to `Astro.locals` to render your 500 page.