Add pre-build step for docs-migration script

[adierkens]

Fixing an issue with the migration status workflow: https://github.com/primer/react/actions/runs/18298011773/job/52100289103

Builds the docgen package prior to the workflow running

Rollout strategy

• None; if selected, include a brief description as to why
  • Changes are to internal tools only

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Latest commit: ca5d655

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Pull Request Overview

This PR fixes a build issue in the migration status workflow by adding a pre-build step for the docgen package before running the migration script.

• Adds a build step for the @primer/doc-gen workspace to ensure dependencies are built before the migration script runs
• Resolves a workflow failure where the migration script couldn't find the required built assets