add suggest changes link

Author: mbostock

docs/observable.tmpl

@@ -46,7 +46,15 @@
       </div>
     </header>
     <main style="min-height: calc(100vh - 20rem);"></main>
-    <footer style="width: 100vw; margin: 12rem calc((100% - 100vw) / 2) -16px; padding-bottom: 16px; background-color: var(--theme-foreground); color: var(--theme-background); font: 12px/normal var(--sans-serif);">
+    <div style="margin: 12rem 0 2rem; font: 500 14px/normal var(--sans-serif);">
+      <a id="edit-page" href="https://github.com/observablehq/notebook-kit">
+        ✎ Suggest changes to this page
+      </a>
+    </div>
+    <script type="module">
+      document.querySelector("#edit-page").href += `/edit/main/docs${location.pathname + (location.pathname.endsWith("/") ? "index" : "")}.html`;
+    </script>
+    <footer style="width: 100vw; margin: 2rem calc((100% - 100vw) / 2) -16px; padding-bottom: 16px; background-color: var(--theme-foreground); color: var(--theme-background); font: 12px/normal var(--sans-serif);">
       <div style="display: flex; flex-wrap: wrap; max-width: var(--max-width); padding: 2rem 1rem; margin: auto; gap: 2rem;">
         <div>
           <a title="Observable" target="_blank" href="https://observablehq.com/" style="color: inherit;">