Document Groundwork CSS import order

[krowvin]

Summary

• move the documented Groundwork stylesheet import from App.jsx to main.jsx
• show the application stylesheet first and Groundwork CSS second so component styles have predictable precedence
• use the public @usace/groundwork/groundwork.css export across the documentation landing page and getting-started guides
• remove repeated stylesheet imports from component examples

Closes #191

Checklist

• I updated docs/examples if the component API or behavior changed
• I added screenshots for UI changes when useful
• I applied exactly one version label: patch-bump, minor-bump, or major-bump

Release impact

patch-bump is appropriate because this corrects existing setup documentation without changing the component API.

Validation

• npm test -- --run (2 files, 8 tests)
• npm run lint
• Prettier check on all changed files
• npm run build-lib
• npm run build-docs (docs assets built; the existing route-extraction step reports the missing direct @babel/parser development dependency addressed by Fix Vite 8 library React imports #300)
• packed Groundwork and installed it in a clean React 19/Vite 8 consumer using the documented import order
• verified the production build resolves the public CSS export and Groundwork's 2rem search-input padding wins over a deliberately conflicting app rule

[github-actions]

Please set a versioning label of either major-bump, minor-bump, or patch-bump to the pull request.