Add documentation validation to rhdh-e2e-test-utils/docs/ with CI integration:

• Ensure VitePress build (yarn build) succeeds
• Install markdown-link-check for broken link detection (internal & external)
• Add validation scripts to docs/package.json
• Update GitHub Actions workflow to run docs validation

Validation Checks:

• VitePress build succeeds (yarn build)
• No broken internal links
• No broken external links

Acceptance Criteria:

• yarn build runs successfully in CI
• markdown-link-check validates internal/external links
• Script added: lint:links
• Integrated into pre-commit
• CI runs validation on docs/ changes
• Clear error reporting for documentation issues