Add glossary to discuss deployments

What does this MR do and why?

This MR adds a new page under the GitLab development documentation that describes the vocabulary we (intend to) use in the Ops section.

The glossary is motivated by the work on Deployment management.

As agreed in gitlab-com/www-gitlab-com!84513 (closed) we would like to have this content in the docs, instead of the handbook.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.