write Smart contract documentation

Copy pasta from user feedback:

I've started the process of learning about MASSA and evaluating strategies for the POC. Some feedback on the MASSA Smart Contract docs:

• I suggest adding a section titled something like "Migrating from Solidity".. or "Comparison with Solidity" that goes through a non-trivial Solidity example and how to do something similar with MASSA.

The majority of devs with smart contract experience are most likely coming from a Solidity background and may not have used WASM before.

• A section that outlines how to structure a smart contract and basic features like:

  • What determines visibility of a function? (i.e. public, internal, private in Solidity)
  • How to pass constructor arguments
  • Functions with multiple args and types
  • Sending value (i.e. payable in Solidity)
  • Error handling, throwing errors
  • Inheritance, interfaces
  • Imports
  • etc.

Perhaps this is all in the WASM docs and it's the same, but it would be worthwhile to repeat the basics and describe where things are different.

I was pretty confused after looking at the docs. The Trading Bot video was helpful but it's over an hour of content and and not structured like a tutorial. It's also a year old and I'm sure much has changed since.

Review the existing Smart Contract and WASM documentation, using the Trading Bot video as background and the listed Solidity comparison topics as the scope. Done means the documentation explains a non-trivial Solidity migration and core contract concepts, including visibility, constructors, arguments, value transfer, errors, inheritance, interfaces, and imports.