Add C++ concept entry for destructors

[ho-ssain]

Description

Introduce a new entry that explains destructors in C++, including their syntax, usage, and examples for proper resource management.
Created file-: docs/content/cpp/concepts/destructors/destructors.md

Issue Solved

Closes #5981

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.