[Term Entry]: Add Five Second Test

[AnamolR]

Description

This PR adds a new UX research entry titled Five Second Test to the uiux/concepts section.

New folder: content/uiux/concepts/five-second-test/
New image: media/five-second-test-flow.png
Markdown file: five-second-test.md includes a definition of the method, when to use it, step-by-step explanation, example questions, benefits, tools, and limitations.

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

Issues Solved

Closes #6590

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.