Use case directory structure

[arshadkazmi42]

Fixes #237

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Move USECASES in a seperate directory
• Each use case is moved to its own file
• README.md added in use-case directory to add table of contents of all the use case

If you have questions, please send an email to Sendgrid, or file a Github Issue in this repository.

[SendGridDX]

All committers have signed the CLA.

[thinkingserious]

Hello @arshadkazmi42,

Thanks again for the PR!

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[thinkingserious]

Hello @arshadkazmi42,

Thanks again for the PR!

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