docs: add bun run dev command to template readme for create-next-app (#55504)
## For Contributors
### 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
### What?
add bun run dev command to template readme for create-next-app