Add justify-content term entry for flexbox

[ritikaa1802]

Description

This PR adds a new documentation entry for the justify-content CSS property under the Flexbox concept.

The entry explains what justify-content does, includes a syntax section with available values, and provides a simple example to help learners understand how flex items are aligned along the main axis.

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Issue Solved

N/A

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Type of Change

• Adding a new entry

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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.

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