Added bucket_size() in unordered_map in cpp

[anuj123upadhyay]

Description

This PR adds a new Docs entry for C++ bucket_size() under content/cpp/concepts/unordered-map/terms/bucket-size/bucket-size.md
the entry explain the uses of the bucket_size() for unordered_map in C++ , including:

• A clear description of its purpose
• A dedicated ##Syntax section covering all overloads
• Example use cases in plain code
• A live Codebyte example using codebyte/cpp for interactive execution

Issue Solved

Closes #7140

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.

[mamtawardhani]

Hey @anuj123upadhyay, can you please remove the other files from the branch and keep only the required one?