fix(ci); add sitemap_index with core api

Author: zoobestik

sitemap_index.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+    <sitemap>
+        <loc>https://kotlinlang.org/sitemap.xml</loc>
+    </sitemap>
+    <sitemap>
+        <loc>https://kotlinlang.org/api/core/sitemap.xml</loc>
+    </sitemap>
+</sitemapindex>