docs(cpp): add reserve() entry for unordered_map #7144

[Mohit5Upadhyay]

Description

This PR adds a new documentation entry for the reserve() method in std::unordered_map, as requested in issue #7144.

It includes:

• A short explanation
• Syntax section
• Example
• Codebyte with compilable code

Issue Solved

Closes: #7144

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.

[Mohit5Upadhyay]

Hi @mamtawardhani 👋,

I’ve created the new entry for reserve() under unordered_map in C++ as per the issue description (#7144).

Kindly review the PR when you get a chance. If any changes or tweaks are needed, please let me know — happy to update accordingly.

Thanks! 😊

[mamtawardhani]

Looks good for a second review! 🚀