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

[ksuraev]

Description

This PR adds a new documentation entry for the C# Sign() method in maths-functions.
The new file can be found at content/c-sharp/concepts/math-functions/terms/sign/sign.md

Issue Solved

Closes #7952

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.

[mamtawardhani]

Looks good for a second review! 🚀