Use the releases feature to "package" versions of this training (and others)

[luisesanmartin]

According to GitHub's documentation about releases, we can set a release to package files as if we were releasing the version of a software.

I think we can consider using this feature to package up a version/edition of this and other trainings we've developed. This would improve the provenance and archiving of the materials we use and tag them in a persistent way. Right now we use branches for this, and though they seem to work acceptably, the materials sitting in a branch after the training took place are still prone to be modified by subsequent commits.