docs: add Interactive Apps guide (#94020)
### What?
Adds the Interactive Apps guide
(`docs/01-app/02-guides/interactive-apps.mdx`) with a companion
[Taskboard app](https://github.com/vercel-labs/async-react-demo).
### 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](https://github.com/vercel-labs/async-react-demo).
The `plain` branch is the starting point, `main` has all 8 steps
applied.
---------
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
