add doc links

x-pack/filebeat/docs/inputs/input-http-endpoint.asciidoc

@@ -230,7 +230,7 @@ In certain scenarios when the source of the request is not able to do that, it c
 [float]
 ==== `program`
 
-The normal operation of the input treats the body either as a single event when the body is an object, or as a set of events when the body is an array. If the body should be treated handled differently, for example a set of events in an array field of an object to be handled as a set of events, then a CEL program can be provided through this configuration field. No CEL extensions are provided beyond the function in the CEL standard library. CEL optional types are supported.
+The normal operation of the input treats the body either as a single event when the body is an object, or as a set of events when the body is an array. If the body should be treated handled differently, for example a set of events in an array field of an object to be handled as a set of events, then a https://opensource.google.com/projects/cel[Common Expression Language (CEL)] program can be provided through this configuration field. No CEL extensions are provided beyond the function in the CEL https://github.com/google/cel-spec/blob/master/doc/langdef.md#standard[standard library]. CEL https://pkg.go.dev/github.com/google/cel-go/cel#OptionalTypes[optional types] are supported.
 
 [float]
 ==== `response_code`