Add Secure Glossary of Terms

Why is this change being made?

An outcome of a recent Secure Stage Retrospective was to create a Secure Glossary of Terms.

The idea of this glossary is to help the team communicate more effectively with each other and to help new people to the stage grasp the concepts faster.

This resolves issue gitlab-org/gitlab#215382 (closed).

Outstanding questions

• Have I linked to the Glossary correctly?
• Should we capitalize words like Finding?
• When using words defined elsewhere in the glossary, should I link to them?

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the change relates to any part of the project other than updates to content and/or data files (e.g. team.yml) please make sure to ping @gl-static-site-editor in a comment for a review and merge.
  • If the DRI for the page/s being updated isn't immediate clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies

For help with failing pipelines reach out in #mr-buddies in Slack

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👀 glossary-of-terms 👀 (note: may need to deploy review app in pipeline)