add join method concept for C#

[jkayk]

Description

Adds the concept entry for the Join() method in C#.

Issue Solved

Closes #5566

Type of Change

• Adding a new entry

Checklist

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