Update glossary

What does this MR do?

Updates the runner docs glossary with research-based terms.

Why was this MR needed?

We have some recommended, research-based terms for GitLab Runner components. But we haven't officially decided to accept and document them. We're writing them down here in the meantime to help align terminology within the runner teams.

What's the best way to test this MR?

What are the relevant issue numbers?