Fix - Add missing information on custom model data to docs

[vectrixdevelops]

Overview

Description: Adds some documentation on custom model data in ItemStackIcon.

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Review Request Checklist

• [x] Your code follows the style guidelines of this project.
• [x] I have performed a self-review of my code.
• [x] I have tested this change myself. (If applicable)
• [x] I have made corresponding changes to the documentation. (If applicable)
• [x] The branch name follows the projects naming conventions. (e.g. feature/add-module & bugfix/fix-issue)

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