Add key terms to the glossary in the reference section

[jcohen02]

Comments from the initial editorial checks carried out as part of the peer review process (see carpentries-lab/reviews#15) highlight that key terms need to be added to the glossary (see the Reference section).

The editor highlights that terms and their definitions can be added here or, where terms are already provided in the Carpentries Glosario glossary, the term can be added and linked to Glosario.