Syntax for invoking clauses and alignment with terminology in CLI&API

[jeromesimeon]

There is a not-really-documented feature in Ergo to call a contract. The place where this appears in the documentation currently is in the REPL:

http://localhost:3000/docs/next/ergo-repl.html#calling-contracts

What should happen:

• [ ] Test and clean up that feature when used within Clauses themselves
• [ ] The syntax should be aligned with the one in the Ergo/Cicero CLI and API:
  • init -> initialize
  • call -> invoke
  • send -> trigger
• [ ] Document that feature in the main Ergo Guide