Add css box sizing doc

[ChanduTelaga]

Description

This PR improves the existing CSS box-sizing documentation by fixing Markdown formatting issues, refining explanations, and improving clarity.
It enhances readability while keeping the content accurate and consistent with Codecademy Docs style guidelines.

Issue Solved

N/A

Type of Change

• Editing an existing entry (fixing a typo, bug, issues, etc)
• Updating the documentation

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.

[mamtawardhani]

Closing this PR as it has H2 for many things which don't follow the Codecademy Standards, and also this PR doesn't trace back to any issue.