[Issue]: Establish a better STYLE_GUIDE.md

[soramicha]

Branch Name

fix/75-style

Task

Currently we've been working on filtering, sorting, pagination, etc. But we've all been writing code in our own ways making the code a bit inconvenient.

Especially variable naming or approaches in coding specific parts, we want to clearly establish a better STYLE_GUIDE.md. Please do research and in the STYLE_GUIDE.md, add any industry standard ways we can start applying in order to get cleaner and consistent code.

Right now I've been quite lenient with the way we've been coding but since this is a big project, I think it's best to start having everyone stick to our STYLE_GUIDE.md whenever we write code now!

Acceptance Criteria

• Clear and detailed STYLE_GUIDE.md that's easy for everyone to follow and follows industry standards!

Checklist

• I have checked for duplicate issues.
• I have assigned the issue to the correct project board.
• I have tagged the issue properly.

Code of Conduct

• I agree to follow this project's Code of Conduct