docs: expand CSS rendering pipeline answers

[splincode]

Summary

• expand all nine answers in the CSS rendering pipeline subsection
• explain FOUC/font loading, CSSOM, render-tree semantics and the critical rendering path
• clarify layout/paint/compositing, render-blocking resources, scroll performance and transition: all trade-offs
• add practical examples, edge cases and interview takeaways without bloating short answers

Scope

• continues Расширить полные ответы во всех разделах #49
• CSS-only docs change in src/pages/css/index.md
• completes the CSS rendering pipeline subsection
• Modern CSS and CSS layout/component tasks stay for follow-up PRs

Verification

CI should validate:

• npm run format:check
• production build

Part of #49