add description for rpk shadow (for index page)

Author: micheleRP

modules/reference/pages/rpk/rpk-shadow/rpk-shadow.adoc

@@ -1,4 +1,5 @@
 = rpk shadow
+:description: These commands let you manage shadow links for disaster recovery.
 // tag::single-source[]
 
 Manage Redpanda shadow links.
@@ -14,6 +15,7 @@ rpk shadow [command] [flags]
 
 == Flags
 
+
 [cols="1m,1a,2a"]
 |===
 |*Value* |*Type* |*Description*