Edited the readme file to reflect changes in the setup procedure for bun.

[HeidiTao]

Description

Edited the readme file to reflect changes in the setup procedure for bun. Removed the frontend and backend sections since both setup can be completed by running bun. Changed the command lines to code block.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[railway-app]

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