[integer] Improve BigUInt multiplication by implementing Karatsuba algorithm

[forfudan]

This pull request introduces enhancements multiplication of BigUInt by implementing the Karatsuba algorithm.

New BigUInt Functionality:

• Implement Karatsuba algorithm for multiplication.
• Implement slices addition and multiplication.
• src/decimojo/biguint/biguint.mojo: Added a new method scale_up_by_power_of_billion to efficiently scale a BigUInt in-place by powers of a billion, enabling faster operations for specific use cases.
• Removed the raises keyword from the __iadd__ method as it no longer throws exceptions.

Benchmarking Enhancements:

• benches/biguint/bench_biguint_multiply.mojo: Added benchmarks for new cases involving extremely large number multiplications (9000 digits * 9000 digits and 36000 digits * 36000 digits). Updated terminology for clarity. [1] [2]
• benches/biguint/bench_biguint_multiply_complexity.mojo: Introduced a comprehensive benchmark for analyzing multiplication time complexity across varying word sizes, including detailed logging and performance analysis.

Configuration and Documentation Updates:

• pixi.toml: Added a new doc command to generate documentation with validation and diagnostics, improving project maintainability.

Miscellaneous Improvements:

• benches/biguint/bench_biguint_multiply.mojo: Configured Python's sys module to handle extremely large integers by setting int_max_str_digits to 100,000.
• Fix a bug for BigUInt comparison where there are leading zero words.