docs: expand EmptyState component guide

[Fred0104]

Summary

• Expanded docs/components/EmptyState.md with clearer usage guidance and prop behavior.
• Documented the illustration variant to Tailwind class mapping.
• Added a current usage table for the contracts, milestones, and reputation pages.
• Clarified the role="region" + useId heading association and variant testing checklist.

Closes #103

Validation

• git diff --check
• npm run build
• npm test -- --runInBand (20 suites, 125 tests passed)

Notes

Docs-only change. No runtime behavior changed.

[mikewheeleer]

clean docs, thanks for expanding the EmptyState guide. in it goes