Add margin block term

[mawowo67]

Description

Added a new term entry for margin-block in CSS under:

content/css/concepts/margins/terms/margin-block/margin-block.md

This entry includes:

• A description of the property.
• Syntax section with details for <start> and <end>.
• Example section with CSS code block demonstrating usage.
• Notes explaining the advantages of using logical properties and supporting different writing modes.

Issue Solved

Closes #8216

Type of Change

• Adding a new entry
• 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.