[decimal] Rename floor_divide to truncate_divide to avoid ambiguity

[forfudan]

This pull request involves renaming the floor_divide function and all related references to truncate_divide across multiple files. The changes ensure consistency in the naming convention and update the logic accordingly.

Renaming and Updating Functions:

• benches/decimal/bench.mojo: Renamed bench_floor_divide to bench_truncate_divide in the import statements and function calls.
• benches/decimal/bench_truncate_divide.mojo: Renamed from bench_floor_divide.mojo and updated all instances of run_benchmark_floor_divide to run_benchmark_truncate_divide.
• src/decimojo/__init__.mojo: Updated import statement from floor_divide to truncate_divide.
• src/decimojo/decimal/arithmetics.mojo: Renamed floor_divide function to truncate_divide and updated the corresponding logic in the modulo function.

Updating Decimal Struct:

• src/decimojo/decimal/decimal.mojo: Updated all instances of floor_divide to truncate_divide in the __floordiv__, __mod__, __rfloordiv__, __rmod__, __ifloordiv__, and __imod__ methods. Added docstrings to reflect the changes.