[Concept Entry] Add string functions in SQL

[PragatiVerma18]

Description

Add a new entry for String Functions in SQL. The entry exists as content/sql/concepts/string-function/string-function.md.

Issue Solved

Closes #5480

Type of Change

• Adding a new entry
• Editing an existing entry (fixing a typo, bug, issues, etc)
• Updating the documentation

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.

[Radhika-okhade]

Hey! @PragatiVerma18 The entry looks good to be merged.

[github-actions]

👋 @PragatiVerma18
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