[Edit] Placeholders: Context and Types

[AnamolR]

Description

This PR updates the existing Placeholders entry in the uiux/concepts/ section to include two new content sections: Purpose in Design and Types of Placeholders.

Markdown file: placeholders.md
These additions clarify why placeholders are used in UX workflows and outline common types such as text, image, UI component, and loading (skeleton screen) placeholders. The updates enhance the entry’s value for learners working with wireframes, prototyping, and design systems.

Issues Solved

Closes #6601

Type of Change

Editing an existing entry (fixing a typo, bug, issues, etc)

Checklist

• All writings are my own.
• My edits follow 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 the updated content 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.