[C++] Base class pointers

[linuxuxs]

Description

I have enhanced the documentation for classes in c++.
What I added: base class pointers (with an example).
Where I added: main/content/cpp/concepts/classes/classes.md
Why I added: Initially I wanted to create a new topic, but after noticing that base class pointers were missing, I added them myself.

Issue Solved

N/A

Type of Change

• 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.