docs: add Interactive Apps guide

[aurorascharff]

What?

Adds the Interactive Apps guide (docs/01-app/02-guides/interactive-apps.mdx) with a companion Taskboard app.

Why?

There is no end-to-end guide that shows how to coordinate pending work across Server Functions, transitions, optimistic UI, and caching in a single app.

How?

The guide walks through building a task management board in 8 steps, from a plain Server Component app to one with optimistic UI, streaming, caching, and prefetching:

1. Stream slow data with Suspense — promise-passing to split reads into independent boundaries
2. Respond instantly to a toggle — useOptimistic + startTransition for a priority cycle
3. Filter with pending feedback — useTransition + data-pending for label chips, action props for ChipGroup
4. Add comments with instant feedback — useOptimistic([]) for a pending-only comment list
5. Move cards between columns — optimistic reducer for drag-and-drop status updates
6. Manage form lifecycle with useActionState — isPending, key-based reset, double-transition for dialog close
7. Signal pending deletion to a parent — useOptimistic(false) + data-pending CSS attribute
8. Make repeat navigation instant — use cache, cacheTag, updateTag, unstable_prefetch

The companion app lives at vercel-labs/async-react-demo. The plain branch is the starting point, main has all 8 steps applied.

[github-actions]

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Commit: 22918c5

[Copilot]

Pull request overview

Adds a new App Router guide for building interactive applications with coordinated pending states across Server Functions, transitions, optimistic UI, Suspense streaming, caching, and runtime prefetching.

Changes:

• Introduces an end-to-end Taskboard walkthrough with eight progressive interaction patterns.
• Covers client/server coordination patterns such as useOptimistic, useTransition, useActionState, data-pending, use cache, cacheTag, updateTag, and unstable_prefetch.
• Links to a companion demo and related Next.js documentation.

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[icyJoseph]

Looks good to me, there's a few things I want to try out myself after reading the guide, often a good sign!

I am ready to approve the guide, but in terms of discoverability, no other document points to this guide, https://nextjs.org/docs/app/getting-started/mutating-data should point as Next Steps, or in prose somewhere relevant