Add Kubernetes Observability workshop to Dev Envagelism projects handbook and GitLab Learn

Why is this change being made?

Document the Kubernetes Observability learning resource from https://gitlab.com/gitlab-com/marketing/corporate_marketing/corporate-marketing/-/issues/6896 in the Developer Evangelism handbook and GitLab Learn.

@abuango This is a thought leadership piece; maybe we iterate in the future on a central page, or have GitLab pages render all pieces.

I don't know who the DRI is for GitLab Learn, but since the workshop is maintained by our team, amongst 4 other entries on the page, I'd say we can approve/merge on our own.

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
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