Add Example in Secure Airthmetic: using Oblivious Offload

[Tom-Finke]

• New Docs Section: "Tutorials"
• Explain how a simple cient - server architecture can work with OblivousOffload
• Add Note in README.md to point users to ObliviousOffload "To realize simple client server archtitectures with SecureArithmetic, please have a look at ObliviousOffload".

[Tom-Finke]

ToDo: Add low-entry level tutorial with code from the perspective of a SecureArithmetic user who wants to implement a trivial computation where both server and client have some information that the other should not see. Explain which steps have to be performed and why.

1. Describe How the setup will look
2. Describe necessary steps to achieve that setup
3. Describe how SecAr. code must be modified to work with OblOffl.

[Tom-Finke]

Documenter might be able to execute code during docs compilation. This would be ideal to verify the docs are still correct even after an API change.