Update Airbyte Custom Source/Destination documentation

[anzelpwj-rev]

The documentation for the custom source and custom destination does not describe how to get the destination ID (which I had to find out via https://github.com/airbytehq/airbyte/discussions/40699#discussioncomment-9951835). Can we update the documentation with instructions on how to find this information (via the postgres DB)?

I'm happy to make the change but am unsure where the true documentation lies. https://github.com/airbytehq/terraform-provider-airbyte/blob/main/docs/data-sources/source_custom.md and https://registry.terraform.io/providers/airbytehq/airbyte/latest/docs/resources/source_custom are not in sync.