examples: add StackBlitz preview to README.md (#73546)
### Description
Follow up #35349.
1. Update the [example
template](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)
with [StackBlitz](https://stackblitz.com/) preview
2. Add `preview live with StackBlitz` to the examples
3. Remove the below old StackBlitz preview section from the examples
```md
## Preview
Preview the example live on [StackBlitz](http://stackblitz.com/):
[](https://stackblitz.com/github/vercel/next.js/tree/canary/examples/active-class-name)
```
CC: @samcx
### Improving Documentation
- [x] Run `pnpm prettier-fix` to fix formatting issues before opening
the PR.
- [x] Read the Docs Contribution Guide to ensure your contribution
follows the docs guidelines:
https://nextjs.org/docs/community/contribution-guide
### Adding or Updating Examples
- [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
