Add C++ unordered_set bucket_count() term entry

[ylevental]

Description

This PR adds a new term entry for the bucket_count() method under C++ unordered_set. The entry documents the syntax, parameters, return value, and includes comprehensive examples demonstrating basic usage, bucket growth monitoring, and hash distribution analysis with a runnable codebyte.

The entry follows the established pattern from unordered-map/bucket-count but adapted for set semantics (elements hashed directly, not key-value pairs). All examples use practical scenarios like tracking programming languages and colors to illustrate bucket allocation strategies.

New term entry: content/cpp/concepts/unordered-set/terms/bucket-count/bucket-count.md

• Syntax and return type documentation
• Two examples: basic usage and monitoring bucket growth during insertions
• Runnable codebyte demonstrating hash distribution analysis across buckets
• FAQ covering differences from size(), performance implications, and rehashing behavior

Issue Solved

Closes #8052

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]

The PR looks good for a second review! 🚀