add postgresql functions concept

[hakdo99]

Description

Add a concept entry about PostgreSQL functions. The new file is functions.md.

Issue Solved

Closes #4589

Type of Change

• Adding a new entry

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.

[SaviDahegaonkar]

Hey @NelsonSanti , Can I know if you have implemented the suggested changes so I can make it available for a second review?

[NelsonSanti]

Hello @SaviDahegaonkar it seems you were mistaken, This job is not assigned to me. No worries and thank you!

[SaviDahegaonkar]

Hello @SaviDahegaonkar it seems you were mistaken, This job is not assigned to me. No worries and thank you!

Yeah, I am really sorry for the misunderstanding.

[SaviDahegaonkar]

Hey @hakdo99 , Can I know if you have implemented the suggested changes so I can make it available for a second review?

Thanks, Savi

[github-actions[bot]]

👋 @hakdo99 You have contributed to Codecademy Docs, and we would like to know more about you and your experience. Please take a minute to fill out this four question survey to help us better understand Docs contributions and how we can improve the experience for you and our learners. Thank you for your help!

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https://www.codecademy.com/resources/docs/postgresql/functions

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