Link docs /latest/ to /stable/ again

Re-implementing the pattern from https://til.simonwillison.net/readthedocs/link-from-latest-to-stable

Refs #1608

docs/_templates/base.html

@@ -3,4 +3,29 @@
 {% block site_meta %}
 {{ super() }}
 <script defer data-domain="docs.datasette.io" src="https://plausible.io/js/plausible.js"></script>
+<script>
+jQuery(function ($) {
+  // Show banner linking to /stable/ if this is a /latest/ page
+  if (!/\/latest\//.test(location.pathname)) {
+    return;
+  }
+  var stableUrl = location.pathname.replace("/latest/", "/stable/");
+  // Check it's not a 404
+  fetch(stableUrl, { method: "HEAD" }).then((response) => {
+    if (response.status == 200) {
+      var warning = $(
+        `<div class="admonition warning">
+           <p class="first admonition-title">Note</p>
+           <p class="last">
+             This documentation covers the <strong>development version</strong> of Datasette.</p>
+             <p>See <a href="${stableUrl}">this page</a> for the current stable release.
+           </p>
+        </div>`
+      );
+      warning.find("a").attr("href", stableUrl);
+      $("article[role=main]").prepend(warning);
+    }
+  });
+});
+</script>
 {% endblock %}