fixed 404 custom page

src/pages/404.js

@@ -0,0 +1,5 @@
+export default function Custom404() {
+    return <h3>
+      Page Not Found. 404.
+    </h3>
+  }

src/pages/en/404.md

@@ -1,27 +0,0 @@
----
-title: Page not found
-description: The page you're looking for isn't here.
----
-
-## Whoops
-
-Looks like the page you are looking for isn't here.
-
--> Developer content has recently moved to [docs.hiro.so](https://docs.hiro.so/). For more information on the content move, see [this post](https://forum.stacks.org/t/the-evolution-of-the-stacks-documentation-and-a-new-hiro-docs-site/12343) on the Stacks forum. Check for your content at [docs.hiro.so](https://docs.hiro.so/) or ask in Discord if you believe you have reached this page in error.
-
-Try out some of these popular pages:
-
-## Understand Stacks
-
-[@page-reference | grid]
-| /understand-stacks/overview, /understand-stacks/proof-of-transfer, /understand-stacks/testnet
-
-## Write smart contracts
-
-[@page-reference | grid]
-| /write-smart-contracts/overview
-
-## Start mining
-
-[@page-reference | grid]
-| /start-mining/mainnet, /start-mining/testnet