add landing page to redirect to OpenAPI spec

Author: Blargian

docs/use-cases/observability/clickstack/api-reference.md

@@ -0,0 +1,15 @@
+---
+slug: /use-cases/observability/clickstack/api-reference
+title: 'API reference'
+pagination_prev: null
+pagination_next: null
+description: 'Landing page for ClickStack API reference docs'
+doc_type: 'landing-page'
+---
+
+import CloudNotSupportedBadge from '@theme/badges/CloudNotSupportedBadge';
+import {Button} from "@clickhouse/click-ui/bundled";
+
+<CloudNotSupportedBadge/>
+
+For API reference docs, please visit [this page](/docs/clickstack/api-reference)

sidebars.js

@@ -1695,11 +1695,7 @@ const sidebars = {
             },
           ]
         },
-        {
-          type: "link",
-          label: "API reference",
-          href: "https://clickhouse.com/docs/clickstack/api-reference",
-        }
+        "use-cases/observability/clickstack/api-reference"
       ]
     },
   ],