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

• 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.

[SaviDahegaonkar]

Hey @hakdo99 ,
The entry is nicely written, and I have suggested a few changes. Please make them asap.

Thanks,
Savi

[SaviDahegaonkar]

Suggested change

	- 'learn-postgresql'
	- 'learn-postgresql'

Include only those course paths that are already available in the catalog-content.md file. You can add one if you want it to get included in the PR. Here learn-postgresql is not available in the catalog-content file so it makes it invalid to get included in PR.

[SaviDahegaonkar]

Suggested change

	**`Functions`** in PostgreSQL, also known as Stored Procedures, are sets of SQL and procedural commands stored on the database server. They enable us to perform executions within the database within a single function that usually require several commands and round trips.
	**`Functions`** in PostgreSQL, also known as Stored Procedures, are sets of SQL and procedural commands stored on the database server. They enable us to perform executions within the database within a single function that usually requires several commands and round trips.

[SaviDahegaonkar]

Suggested change

	CREATE [OR REPLACE] FUNCTION function_name (parameters)
	RETURNS return_type AS $$
	DECLARE
	< variable_declaration >
	[...]
	BEGIN
	< function_body >
	[...]
	RETURN { variable_name | value }
	END;
	$$ LANGUAGE lang_name;
	CREATE [OR REPLACE] FUNCTION function_name (parameters)
	RETURNS return_type AS $$
	DECLARE
	< variable_declaration >
	[...]
	BEGIN
	< function_body >
	[...]
	RETURN { variable_name | value } ;
	END;
	$$ LANGUAGE lang_name;

Added semicolon at line number 32 for correct syntax.