docs: update monorepo documentation to reflect Tailwind v4 migration

[georgewrmarshall]

Summary

Several documentation files still reference the old Tailwind v3 setup after the v4 migration (#842). These should be updated so consumers and contributors get accurate information.

Files to Update

Root README.md

• Update dependency graph (mermaid) to remove design_system_react --> design_system_tailwind_preset edge
• Mark @metamask/design-system-tailwind-preset as deprecated/v3-only in the package list (or defer to chore: deprecate @metamask/design-system-tailwind-preset #981)

CLAUDE.md

• Update package descriptions to note that design-system-tailwind-preset is v3-only/deprecated
• Note that design-tokens now exports tailwind/theme.css for v4 consumers

design-tokens/README.md

• Add a "Tailwind CSS v4" section showing how to use @metamask/design-tokens/tailwind/theme.css
• Update the existing "CSS Variables" section to note that Tailwind v4 consumers should use theme.css instead of styles.css

design-system-react/README.md

• Add setup instructions noting Tailwind v4 and @metamask/design-tokens as requirements
• Link to the migration guide for consumers upgrading from v3

Context

The migration guides (design-tokens/MIGRATION.md and design-system-react/MIGRATION.md) are already comprehensive. This issue covers the README and contributor docs that weren't updated as part of #842.