Custom layout template for docs

Adds plausible analytics, closes #389

Implements banner on latest page, closes #388

docs/_templates/layout.html

@@ -0,0 +1,39 @@
+{%- extends "!layout.html" %}
+
+{% block htmltitle %}
+{{ super() }}
+<script defer data-domain="docs.datasette.io" src="https://plausible.io/js/plausible.js"></script>
+{% endblock %}
+
+{% block footer %}
+{{ super() }}
+<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 = $(
+        `<div class="admonition warning">
+           <p class="first admonition-title">Note</p>
+           <p class="last">
+             This documentation covers the <strong>development version</strong> of <code>sqlite-utils</code>.</p>
+             <p>See <a href="${stableUrl}">this page</a> for the current stable release.
+           </p>
+        </div>`
+      );
+      warning.find("a").attr("href", stableUrl);
+      var body = $("div.body");
+      if (!body.length) {
+        body = $("div.document");
+      }
+      body.prepend(warning);
+    }
+  });
+});
+</script>
+{% endblock %}