docs: add documentation for unordered_set find method

[JayArindamMaity]

Description

I have added a new documentation entry for the .find() method within the C++ unordered_set concept.

This PR creates the file content/cpp/concepts/unordered-set/terms/find/find.md. The entry includes:

• A clear definition of how .find() works.
• Syntax and parameter details.
• A standard code example demonstrating return value checks (found vs. not found).
• A runnable Codebyte example for interactive learning.

Issue Solved

Closes #8028

Type of Change

• [x] Adding a new entry

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.

[CLAassistant]

All committers have signed the CLA.