[Term Entry] C# Math-functions: BigMul()

[ksuraev]

Description

This PR adds a new documentation entry for the C# BigMul() method in maths-functions.
The new file is located at content/c-sharp/concepts/math-functions/terms/bigmul/bigmul.md

Issue Solved

Closes #7947

Type of Change

• Adding a new entry

Checklist

• All writings are my own.
• My entry follows the Codecademy Docs style guide.
• My changes generate no new warnings.
• I have performed a self-review of my own writing and code.
• I have checked my entry and corrected any misspellings.
• I have made corresponding changes to the documentation if needed.
• I have confirmed my changes are not being pushed from my forked main branch.
• I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• I have linked any issues that are relevant to this PR in the Issues Solved section.

[CLAassistant]

All committers have signed the CLA.

[mamtawardhani]

Looks good for a second review! 🚀

[ksuraev]

Looks good for a second review! 🚀

Thank you for the feedback/review! Just two things to note (which I did not make clear either in my commit) -

1. yes Int128/UInt128 were introduced in .NET 7, however, the overloads BigMul(Int64, Int64) and BigMul(UInt64, UInt64) were introduced in .NET 9 which I did not initially phrase well (apologies).
2. The three parameter overloads return the high 64-bits and store the low 64-bits in the out parameter. E.g. public static long BigMul(long a, long b, out long low); from .NET docs
   Not sure if you/a maintainer would prefer to make these changes or me :)