[docs] code, comments and format

[zcy1024]

Description

• Standardize code indentation format.
• In the current SDK, there are some places where the fromHex function does not need to be called.
• Some locations require the use of the original package ID, and comments have been added to make it clearer for beginners.
• Design.md: Explicitly specifying the BCS type is not required; otherwise, you would need to add use sui::bcs::BCS;.