docs(examples/with-mobx-state-tree-typescript): add 'Open in StackBli…

…tz' button (vercel#25926)

Add `Open in StackBlitz` button to `examples/with-mobx-state-tree-typescript` now that it was fixed in vercel#25589.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [x] Example(s) updated
- [ ] Telemetry added. In case of a feature if it's used or not.

## Documentation / Examples

- [x] Make sure the linting passes

examples/with-mobx-state-tree-typescript/README.md

@@ -10,6 +10,12 @@ The trick here for supporting universal mobx is to separate the cases for the cl
 
 The clock, under `components/Clock.js`, has access to the state using the `inject` and `observer` functions from `mobx-react`. In this case Clock is a direct child from the page but it could be deep down the render tree.
 
+## Preview
+
+Preview the example live on [StackBlitz](http://stackblitz.com/):
+
+[![Open in StackBlitz](https://developer.stackblitz.com/img/open_in_stackblitz.svg)](https://stackblitz.com/github/vercel/next.js/tree/canary/examples/with-mobx-state-tree-typescript)
+
 ## Deploy your own
 
 Deploy the example using [Vercel](https://vercel.com?utm_source=github&utm_medium=readme&utm_campaign=next-example):