Update with-jest packages and docs (#28209)
This PR updates the with-jest example to the latest version of Jest and adds a note about the Jest and RTL testing environments (related to this [discussion](https://github.com/vercel/next.js/discussions/28337)). Also adds minor tweaks to the testing docs.
## 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
- [x] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [x] Make sure the linting passes
