Link to todo.sr.ht GraphQL API docs

todo.sr.ht has a GraphQL API and a page describing it, but neither the
index page nor the api page linked to it.  This patch updates both pages
to link to the GraphQL documentation, as is already the case for
{meta,git}.sr.ht.

index.html

@@ -121,7 +121,7 @@ dt {
     <dd>
       <a href="/todo.sr.ht">User Manual</a>
       &mdash;
-      <a href="/todo.sr.ht/api.md">API Reference</a>
+      <a href="/todo.sr.ht/graphql.md">API Reference</a>
     </dd>
   </div>
   <div class="col-md-4">

todo.sr.ht/api.md

@@ -2,6 +2,9 @@
 title: todo.sr.ht API reference
 ---
 
+**NOTICE**: This is the documentation for the legacy REST API. It is being
+replaced with the [GraphQL API](graphql.md).
+
 The todo.sr.ht API allows you to browse, create, and manage repositories on
 todo.sr.ht programmatically. This API follows the [standard sourcehut API
 conventions](/api-conventions.md).