Added PostgreSQL Constants Concept

[Dan-Tyas]

Description

Added a folder named "PostgreSQL" with a file named "Constants" into the Codecademy/Docs Folder. I have also made an edit to the tags.md file and added the tag "Constants".

Issue Solved

#4533 - [Concept Entry] PostgreSQL constants

Type of Change

• Adding a new entry
• Editing an existing entry
• Updating the documentation

Checklist

• [X] All writings are my own.
• [X] My entry follows the Codecademy Docs style guide.
• [X] My changes generate no new warnings.
• [X] I have performed a self-review of my own writing and code.
• [X] I have checked my entry and corrected any misspellings.
• [X] I have made corresponding changes to the documentation if needed.
• [X] I have confirmed my changes are not being pushed from my forked main branch.
• [X] I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• [X] I have linked any issues that are relevant to this PR in the Issues Solved section.

[CLAassistant]

All committers have signed the CLA.

[ishg-153]

@Dan-Tyas Please make the suggested changes.

Note: If you don't respond with the changes within a week, this PR will be closed and its corresponding issue will be unassigned from you.

[Sriparno08]

Closing this PR since the author is inactive.