docs: Add use case for substitutions

[Swimburger]

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This PR adds a use case to the documentation for using substitutions.
It wasn't easy to find how to do substitutions to answer this StackOverflow question, so I decided to contribute a sample for it.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified