[Term Entry] C++ Strings: capacity

[zamora-carlos]

Description

• Added a new entry for the capacity() method under C++ strings.
• Created a new file at content/cpp/concepts/strings/terms/capacity/capacity.md to document this term and provide examples.

Issue Solved

Closes issue #7132

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.

[CLAassistant]

All committers have signed the CLA.