Update script & documentation to note this is LEARNER certificates only

We used to have one repo for all of our certificates. This repo included the script to generate the certificates, the svg templates for the certificates, and the certificates themselves.

We separated that into two repos:

* https://github.com/carpentries/certification : a private repo that stores all the certificates officially issued by The Carpentries, the svg templates for the certificates, and the script that generates them. This ensures privacy of the certificates that The Carpentries officially awards.
* https://github.com/carpentries/learner-certificates/ (this repo): A public repo that stores the svg template for learner certificates for Data, Library, and Software Carpentry workshops, and the script that generates them.

When we separated these, we simply copied the `certificates.py` script so it includes references to **Instructor** certificates. This should be updated to only refer to **learner** certificates.