docs: expand CSS compositor layer answers

[splincode]

Summary

• expand CSS property-cost and compositor-layer answers with mechanisms, examples and trade-offs
• clarify layout vs paint triggers, transform/opacity animation behavior and CPU/GPU responsibilities
• explain compositor layers, layer promotion and correct will-change usage

Scope

• continues Расширить полные ответы во всех разделах #49
• CSS-only docs change in src/pages/css/index.md
• top/left, expensive visual effects, layout thrashing, FPS and rendering pipeline stay for follow-up PRs

Verification

• npm run format:check
• npm run build

Part of #49