chore(examples): remove `with-styletron` example (#81842)
## Summary
Remove
[with-styletron](https://github.com/vercel/next.js/tree/canary/examples/with-styletron)
example and its references from the repository.
Documentation:
- Remove the Styletron link from the CSS-in-JS examples list in the docs
Tests:
- Remove the Styletron entry from the
turbopack-dev-examples-manifest.json
Chores:
- Delete the examples/with-styletron directory and its contents
## Why?
[styletron](https://github.com/styletron/styletron) package hasn't been
maintained in 2 years.
## 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
cc: @samcx
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
