docs: add value-focused multi-repo workflow example

[corwinm]

Summary

• Add a value-first README section that explains parallel frontend/backend worktree flow.
• Include a concise add -> create -> switch command narrative and static walkthrough frames for GitHub rendering.
• Keep onboarding messaging aligned with the docs site value proposition.

Related

• Companion implementation/spec PRs: specs: define docs value-prop animation change arashi-arashi#126, docs: add interactive hero worktree animation demo arashi-docs#15
• Related issue: Better explain value of arashi and what it does arashi-arashi#92

[corwinm]

Deferring the README/workflow example changes in for now so we can proceed with docs-only updates in .