docs: expand CSS architecture fundamentals

[splincode]

Summary

• expand core CSS Architecture answers with mechanisms, examples, trade-offs and interview takeaways
• cover methodology, design tokens, cascade layers, Shadow DOM styling, BEM and team CSS principles
• clarify how to handle local exceptions without turning them into accidental global conventions

Scope

• continues Расширить полные ответы во всех разделах #49
• CSS-only docs change in src/pages/css/index.md
• frameworks/approaches/theming/isolation/UI kit stay for the next CSS Architecture PR

Verification

• npm run format:check
• npm run build

Part of #49