[New Entry] Add Tree Testing

[AnamolR]

Description

This PR adds a new UX research entry titled Tree Testing to the uiux/concepts section.

New folder: content/uiux/concepts/tree-testing/
New image: media/tree-testing-task.png
Markdown file: tree-testing.md includes a definition of tree testing, use cases, example tools, task-based navigation explanation, and distinctions between moderated and unmoderated testing.

This entry adheres to the established tone, formatting, and structure used in other UI/UX research and methodology topics.

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.