feat(docs): migrate documentation site into monorepo (#5912)

apps/docs/src/theme/NotFound.js

@@ -0,0 +1,50 @@
+import React from "react";
+import Translate, { translate } from "@docusaurus/Translate";
+import { PageMetadata } from "@docusaurus/theme-common";
+import Layout from "@theme/Layout";
+
+export default function NotFound() {
+  return (
+    <>
+      <PageMetadata
+        title={translate({
+          id: "theme.NotFound.title",
+          message: "Page Not Found",
+        })}
+      />
+      <Layout>
+        <main className="container margin-vert--xl">
+          <div className="row">
+            <div className="col col--6 col--offset-3">
+              <img
+                src="/img/undraw_illustrations/undraw_taken_re_yn20.svg"
+                alt=""
+                height={200}
+                width="auto"
+                style={{
+                  display: "block",
+                  marginBottom: "2rem",
+                  marginLeft: "auto",
+                  marginRight: "auto",
+                }}
+              />
+
+              <h1 className="hero__title">
+                <Translate id="theme.NotFound.title" description="The title of the 404 page">
+                  You found a dead link!
+                </Translate>
+              </h1>
+              <p>
+                <Translate id="theme.NotFound.p1" description="The first paragraph of the 404 page">
+                  Oops, you got a dead link. This means, that the page you're looking for is either not available or has
+                  been deleted. We're sorry for the inconvenience. If you believe, that this page should be reachable,
+                  check your URL for any typos.
+                </Translate>
+              </p>
+            </div>
+          </div>
+        </main>
+      </Layout>
+    </>
+  );
+}