[Concept Entry] SQL String-Function

[codecademy-docs]

Reason/inspiration (optional)

We would like a new entry on the string-function concept under SQL. The entry should go in a new file under docs/content/sql/concepts/string-function/string-function.md.

The entry should include:

• An introduction to the concept
• A Syntax section that provides the syntax for the concept (if applicable)
• An Example section that provides an example demonstrating the concept in use (if applicable)

Please refer to the concept entry template, content standards, and markdown style guide when working on the PR for this issue.

Entry Type (select all that apply)

New Concept Entrty for Existing Topic

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