refactor: update build config for @primer/doc-gen to rolldown

[joshblack]

Closes #

N/A

Changelog

New

N/A

Changed

• Updated @primer/doc-gen to build JavaScript and declarations with Rolldown.

Removed

• Removed the package Rollup build config and Rollup build dependencies.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; build tooling-only change with no public runtime API change

Testing & Reviewing

Validated as part of the stack with npm run build, npm run type-check, npm run lint, npm run lint:css, npm run format:diff, and npm test -- --run.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

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Latest commit: 82272b3

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. If this doesn't work, you can also use the original workflow here. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

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[Copilot]

Pull request overview

This PR refactors the @primer/doc-gen build pipeline by replacing the Rollup-based configuration with a Rolldown-based configuration, keeping the package’s build output (dist JS + declarations) aligned with the repo’s newer bundling tooling.

Changes:

• Removed the Rollup build config file from @primer/doc-gen.
• Added a rolldown.config.js that builds ESM JavaScript and emits .d.ts via rolldown-plugin-dts.
• Updated @primer/doc-gen package scripts and devDependencies to use Rolldown instead of Rollup+Babel.

Show a summary per file

File	Description
packages/doc-gen/rollup.config.js	Removes the legacy Rollup build configuration.
packages/doc-gen/rolldown.config.js	Adds Rolldown build config for ESM output + DTS generation.
packages/doc-gen/package.json	Switches build/watch scripts to Rolldown and updates build-related devDependencies.

Copilot's findings

• Files reviewed: 3/4 changed files
• Comments generated: 1

[joshblack]

Applied: moved typescript to runtime dependencies because doc-gen imports it at runtime, and removed unused @babel/runtime now that Babel is no longer in the build pipeline.