Add ERD question on entity representation in SQL category

[ThesllaDev]

Summary of changes

This PR adds a new question to the SQL category, focusing on the representation of entities in an ERD (Entity Relationship Diagram). The question asks about the common shape used to represent entities and tables in an ERD, with an explanation that highlights the use of rectangles to contain the entity name.

Checklist

• [x] I have read and followed the contribution guidelines.
• [x] I have read through the Code of Conduct and agree to abide by the rules.
• [x] This PR is for one of the available issues and is not a PR for an issue already assigned to someone else.
• [x] My PR title has a short descriptive name so the maintainers can get an idea of what the PR is about.
• [x] I have provided a summary of my changes.