Re-open add copy() and copyIndex() function reference documentation

Author: Gijsreyn

docs/reference/schemas/config/functions/overview.md

@@ -645,4 +645,4 @@ The following list of functions create or convert values of a given type:
 [reference]:   ./reference.md
 [resourceId]:  ./resourceId.md
 [sub]:         ./sub.md
-[variables]:   ./variables.md
+[variables]:   ./variables.md