Including two different versions of NGSI-LD OAS release 1.8.1: one for OpenAPI...

Including two different versions of NGSI-LD OAS release 1.8.1: one for OpenAPI version 3.0.3 and one for OpenAPI version 3.1.0. The OpenAPI version 3.0.3 is recommended to be used by third-party development tools as most of them do not support version 3.1.0 yet. The OpenAPI version 3.1.0 is recommended for reviewing the NGSI-LD OAS documentation and operations. The ReDoc visualization application has been chosen as the solution to navigate the NGSI-LD OAS documentation.