Document Scilla Server API

https://github.com/Zilliqa/scilla/wiki/scilla-server-API

Recently I've got this question from a developer.

there are ways to get the contract code for a given contract, but is there an easy way to get all the transitions for that contract?

I explained that using https://scilla-server.zilliqa.com/contract/check is the way to get the transitions.

Maybe this can be documented better.