Created term entry for .append() method for C++ strings

[BrianNazir]

Description

I created a term entry for the C++ strings .append() method. My motivation for doing this was a a part of the kanban project for the "Learn Git & GitHub" course on Codecademy that I took as supplement to my learning during the Full Stack Engineer career path.

Issue Solved

Closes #7093 [Term Entry] C++ Strings: append()

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.