docs: redesign documentation site

[Maksim-Burtsev]

Summary

• replace the basic docs homepage with the approved visual design
• add the interactive six-step migrator flow and truth-checked guardrail examples
• switch the docs build to pinned Zensical and keep GitHub Pages deployment on master
• add responsive light/dark styling, Lucide assets, and keyboard-accessible interactions

Validation

• uv lock --check
• make lint
• make docs
• make test-all (214 regular tests + 12 cluster tests)
• browser QA: light and dark themes, pointer and keyboard step selection, links, overflow, and console logs