feat: add node version disclaimer

Author: FilipPyrek

docs/src/content/docs/getting-started/add-buttonize-to-existing-project.mdx

@@ -20,6 +20,8 @@ Make sure to first sign up at [app.buttonize.io](https://app.buttonize.io) and c
 
 [`buttonize`](https://github.com/buttonize/buttonize) package contains Buttonize CDK Constructs and CLI.
 
+Node.js version >= 20 is required.
+
 <Tabs>
   <TabItem label="npm">
 

docs/src/content/docs/getting-started/build-your-first-app.mdx

@@ -20,6 +20,8 @@ Make sure to first sign up at [app.buttonize.io](https://app.buttonize.io) and c
 
 Clone starter project, install dependencies and start Buttonize local development.
 
+Node.js version >= 20 is required.
+
 <Tabs>
 	<TabItem label="npm">