Docs: Update addon-test coverage exclude details

[kylegach]

Closes #29613

What I did

See: title

Checklist for Contributors

Testing

N/A - docs-only minor update

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli-storybook/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

Greptile Summary

Updates test coverage documentation to clarify how to properly exclude story files and Storybook build output from Vitest coverage reports.

• Added exclude pattern for Storybook build output directory in code/storybook-static to coverage configuration
• Updated Vitest version reference from 2.2.0 to 3.0.0
• Improved readability by formatting exclude patterns on separate lines
• Added clarification that coverage configuration must be done in user's Vitest config file

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[greptile-apps]

style: Consider adding additional common story file patterns that were previously included: '/.story.', '/stories.', '**/story.'

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