Add netlify to README, intro.md and motd.txt

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

netlify/intro.md

@@ -0,0 +1,13 @@
+<!--
+SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+
+SPDX-License-Identifier: CC-BY-SA-4.0
+-->
+
+:::warning
+What you see is an UI-Test! It's filled with dummy data, not connected to a backend and no data will be saved.
+:::
+
+![HedgeDoc Screenshot](/mock-backend/public/screenshot.png)
+
+[![Deployed using netlify](https://www.netlify.com/img/global/badges/netlify-color-accent.svg)](https://www.netlify.com)

netlify/motd.txt

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+This demo is hosted by netlify.
+Please check their privacy policy (https://netlify.com/privacy) as well as ours (https://hedgedoc.org/privacy-policy).

netlify/motd.txt.license

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+SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+
+SPDX-License-Identifier: CC0-1.0

netlify/patch-intro.js

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+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+const { copyFile } = require('fs/promises');
+
+console.log("Patch intro.md to include netlify banner.")
+copyFile("netlify/intro.md", "public/mock-backend/public/intro.md")
+  .then(() => console.log("Copied intro.md"))
+  .catch((error) => {
+    console.error(error);
+    process.exit(1);
+  })
+
+console.log("Patch motd.txt to include privacy policy.")
+copyFile("netlify/motd.txt", "public/mock-backend/public/motd.txt")
+  .then(() => console.log("Copied motd.txt"))
+  .catch((error) => {
+    console.error(error);
+    process.exit(1);
+  })