with-mux-video: move to app router and update packages (#62297)
## App Router
Moves the `with-mux-video` example to idiomatic App Router, with goodies
like
* server component data fetching
* server actions
* layouts
* route groups
* loading UI
## Mux Dependencies
* @mux/mux-node 7 -> 8
* @mux/mux-player-react 1 -> 2
* @mux/upchunk + custom UI -> @mux/mux-uploader
## In other news...
* Fleshed out the README
* Updated imagery
* Moved from styled jsx to tailwind and lightly updated styles
## Contributor Checklist
* [x] The "examples guidelines" are followed from our contributing doc
https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md
* [x] Make sure the linting passes by running `pnpm build && pnpm lint`.
See
https://github.com/vercel/next.js/blob/canary/contributing/repository/linting.md
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Co-authored-by: Sam Ko <sam@vercel.com>
