Add information that EIP ressource also export the EIP ID

[CHS-OTC]

https://registry.terraform.io/providers/opentelekomcloud/opentelekomcloud/latest/docs/resources/networking_floatingip_v2
https://registry.terraform.io/providers/opentelekomcloud/opentelekomcloud/latest/docs/resources/vpc_eip_v1
https://github.com/opentelekomcloud/terraform-provider-opentelekomcloud/blob/devel/docs/resources/vpn_gateway_v5.md
also some connected information in
https://github.com/opentelekomcloud/terraform-provider-opentelekomcloud/blob/devel/docs/resources/vpn_connection_v5.md

Most OTC API references EIPs by their public IP.
But the Enterprise VPN API uses the EIP ID and not IP.
The OTC TF provider already provides the IDs but that is not documented.

So please update the docs so that it mentioned that these values are actually available as attributes to be used.
opentelekomcloud_vpc_eip_v1.RESSOURCE_NAME.id
opentelekomcloud_networking_floatingip_v2.RESSOURCE_NAME.id

opentelekomcloud_enterprise_vpn_gateway_v5.RESSOURCE_NAME.eip1[0].id
opentelekomcloud_enterprise_vpn_gateway_v5.RESSOURCE_NAME.eip2[0].id

In addition please also improve the wording in vpn_connection_v5 so that it is more clear that "gateway_ip" requires the EIP ID of one of the EIPs connected to the vpn_gateway_v5.