Add documentation for Email Activity API

[reedsa]

Closes #669

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:

• Updates the USAGE.md and USE_CASES.md for the client
• Updates the use-cases with references to relevant documentation

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.

[Ditofry]

🚀 🚢 Thanks @reedsa

[thinkingserious]

Hello @reedsa,

Thanks again for the PR!

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