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.

---

Issue Solved

N/A

---

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.

[CLAassistant]

All committers have signed the CLA.

[ritikaa1802]

Hi, thanks for the update!
I’ve already signed the CLA — please let me know if you need anything from my side.
Appreciate you rechecking it 🙂