Add Math.SinCos() term to C# math-functions

[ylevental]

Description

This PR adds a new term entry for the Math.SinCos() method to the C# math-functions concept. The entry includes:

• A description of the Math.SinCos() method and its purpose
• A Syntax section with method signature and parameter details
• An Example section demonstrating the method with degree-to-radian conversion
• A Codebyte section with runnable C# code

File created: content/c-sharp/concepts/math-functions/terms/sincos/sincos.md

Issue Solved

Closes #7953

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.