Add build.assetsPrefix to configuration reference (#13834)

Co-authored-by: Armand Philippot <git@armand.philippot.eu>

Author: rururux

src/content/docs/en/reference/modules/astro-config.mdx

@@ -119,6 +119,7 @@ See more about the configuration imports available from `astro:config/server`:
 - [`build.format`](/en/reference/configuration-reference/#buildformat)
 - [`build.client`](/en/reference/configuration-reference/#buildclient)
 - [`build.server`](/en/reference/configuration-reference/#buildserver)
+- [`build.assetsPrefix`](/en/reference/configuration-reference/#buildassetsprefix)
 - [`site`](/en/reference/configuration-reference/#site)
 - [`srcDir`](/en/reference/configuration-reference/#srcdir)
 - [`cacheDir`](/en/reference/configuration-reference/#cachedir)