[Term Entry] C++ Unordered-sets: rehash()

[ylevental]

Description

New term entry for rehash() method under C++ unordered-set at content/cpp/concepts/unordered-set/terms/rehash/rehash.md.

Entry includes:

• Description of the method and its purpose for optimizing hash table performance
• Syntax section with parameters (n: minimum bucket count) and return value (void)
• Example demonstrating bucket count changes and load factor impact
• Codebyte with compilable code showing pre-allocation before bulk insertions

Uses explicit std:: prefixes consistent with the existing unordered-map/rehash entry.

Issue Solved

Closes #8054

Type of Change

• Adding a new entry
• ~~Editing an existing entry (fixing a typo, bug, issues, etc)~~
• ~~Updating the documentation~~

Checklist

• [x] All writings are my own.
• [x] My entry follows the Codecademy Docs style guide.
• [x] My changes generate no new warnings.
• [x] I have performed a self-review of my own writing and code.
• [x] I have checked my entry and corrected any misspellings.
• [x] I have made corresponding changes to the documentation if needed.
• [x] I have confirmed my changes are not being pushed from my forked main branch.
• [x] I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• [x] I have linked any issues that are relevant to this PR in the Issues Solved section.

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