add link to old docs (#2496)

Author: langermank

docs/stories/Introduction.mdx

@@ -1,5 +1,9 @@
 <img src="hero-illustration.svg" alt="" class="mb-3" />
 
+<div className="color-fg-accent border rounded-2 color-bg-accent p-3 color-border-accent-emphasis mb-5">
+  <p className="m-0 h4">Looking for the old docs site? Find it <a className="color-fg-accent" href="https://gh.io/AAlnj9s">here</a>.</p>
+</div>
+
 # Introduction
 
 Our goal is to create a system that enables us to build consistent user experiences with ease, yet with enough flexibility to support the broad spectrum of GitHub websites. This goal is embedded in our design and code decisions. Our approach to CSS is influenced by Object Oriented CSS principles, functional CSS, and BEM architecture.