[Concept Entry] on Postgresql constants

[SaviDahegaonkar]

Description

Added a new concept entry on the Constants concept in PostgreSQL.

Issue Solved

4533

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.