[docs]: add a section for Golden Image producers

# Documentation request

It might be good to add a section for Golden Image producers. In my case I was trying to configure an image according to CIS/STIG requirements for folder mounts. I was able to use packer and it's custom_data_file to provide the necessary configuration information to create the partitions and mount them, but when a system was created from the image the fstab would be reset. It was suggested in the libera chat to add a file in /etc/cloud/cloud.cfg.d which worked. From the documentation it was not clear that it was appropriate to use that folder for this purpose. Adding additional documentation might help others trying to build golden images.

Thanks,
Shane

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*Reported from: https://cloudinit.readthedocs.io/en/latest/explanation/configuration.html*