chore: Add example page link in index

demos/dev/example.html

@@ -39,6 +39,8 @@ graph TB
 
     <script type="module">
       import mermaid from '/mermaid.esm.mjs';
+      import flowchartELK from '/mermaid-flowchart-elk.esm.mjs';
+      await mermaid.registerExternalDiagrams([flowchartELK]);
       async function render(str) {
         const { svg } = await mermaid.render('dynamic', str);
         document.getElementById('dynamicDiagram').innerHTML = svg;

demos/index.html

@@ -18,6 +18,7 @@
     <p>
       Some of these pages have duplicates; some are slow to load because they have so many graphs.
     </p>
+    <p>You can test custom code in the <a href="./dev/example.html">development page</a>.</p>
     <p>
       If you'd like to clean up one of the pages, please feel free to
       <a href="https://github.com/mermaid-js/mermaid/pulls">submit a pull request (PR).</a>