[Term Entry] Usability Testing

[AnamolR]

Description

This PR adds a new UX evaluation method entry titled Usability Testing to the uiux/concepts/ section.

New folder: content/uiux/concepts/usability-testing/
Markdown file: usability-testing.md introduces the concept of usability testing, outlines when and why it’s used in the design process, and details its key components and different types. The entry helps readers understand how usability testing improves design quality through task-based observation and user feedback. This content also complements other research method entries such as user interviews, think-aloud protocol, and A/B testing.

Issues Solved

Closes #6605

Type of Change

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.