[New entry] UNHEX() function in MySQL

[cigar-galaxy82]

Description

Issue Solved

#5153

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.

[SaviDahegaonkar]

Hey @cigar-galaxy82 ,
The entry LGTM! I approve it by my side.

[cigar-galaxy82]

Hi @SaviDahegaonkar I have removed all the errors you can merge it now.

[github-actions]

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