fix: set baseURL by Netlify context for valid sitemaps

Author: Ajay-singh1

netlify.toml

@@ -1,6 +1,13 @@
 [build]
     publish = "public"
+    command = "hugo --baseURL=$BASEURL"
+[context.production.environment]
+  BASEURL = "https://istio.io/"
+[context.prelim.environment]
+  BASEURL = "https://preliminary.istio.io/"
 
+[context.deploy-preview.environment]
+  BASEURL = "$DEPLOY_PRIME_URL/"
 [build.environment]
     # Consult the Docs WG before bumping Hugo.
     # Specifically, the version here should match docker/build-tools/Dockerfile in istio/tools,