[Term Entry] Add Postel's Law

[AnamolR]

Description

This PR adds a new UX design principle entry titled Postel’s Law (also known as the Robustness Principle) to the uiux/concepts/ux-psychology/terms/ section.

New folder: content/uiux/concepts/ux-psychology/terms/postels-law/
Markdown file: postels-law.md introduces the principle, explains its relevance to UX, highlights practical applications in interface and input design, and outlines benefits and limitations.

This entry follows the same tone, structure, and formatting conventions used in other UX laws and cognitive principles.

Issues Solved

Closes #6593

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.