Add term entry: C# Math.CopySign() (#7950)

[jaideepkathiresan]

Description

Created a new term entry for Math.CopySign() in C# under math-functions, adding metadata, syntax details, an example, and a runnable Codebyte in content/c-sharp/concepts/math-functions/terms/copysign/copysign.md.

Issue Solved

Closes #7950

Type of Change

• Adding a new entry
• Editing an existing entry (fixing a typo, bug, issues, etc)
• Updating the documentation

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! 🚀