docs: add a note in 404 page about v1 docs (#9815)

www/apps/book/app/_not-found.mdx

@@ -14,6 +14,8 @@ export const metadata = {
 
 The page you were looking for isn't available.
 
+If you're looking for Medusa v1 documentation, it's been moved to [docs.medusajs.com/v1](https://docs.medusajs.com/v1).
+
 If you think this is a mistake, please [report this issue on GitHub](https://github.com/medusajs/medusa/issues/new?assignees=&labels=type%3A+docs&template=docs.yml).
 
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