X-Ray knowledge graph design document

Why is this change being made?

Adds design document for X-Ray knowledge graph feature.

Related to gitlab-org/gitlab#517117 (closed)

Author and Reviewer Checklist

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• 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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Commits

• X-Ray knowledge graph design document

Adds design document for knowledge graph feature.

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