Document changes to insecure registry support

[baronfel]

Update https://github.com/dotnet/sdk-container-builds/blob/main/docs/RegistryAuthentication.md#using-insecure-registries with the changes from dotnet/sdk#41506:

• SDK_CONTAINER_INSECURE_REGISTRIES env var with a comma-separated list of domains to treat as insecure
• description of what insecure means: HTTP supported when HTTPS is not supported at the domain, HTTPS supported with any cert with no policy errors that is intended for the specific host in question