Add glossary page for Switchboard

Why is this change being made?

As we're working to refine the functionality and scope of Switchboard, we are running into confusion RE: the meaning of specific terms and concepts as they relate to Switchboard. @andlovu raised this idea in https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/2259

Some of this information may eventually be moved to the docs site, but for now this will give the team a place to document and iterate on the definitions of important terms.

I also expect that we will come across gaps/confusion in the process of putting this page and MR together which will help guide where further discussion is needed.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • 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
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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