Write SL doc?

[john-science]

The old documentation should be replaced with something that explains the language, not just homework questions:

Part 1: Language Parsing (ast + parser) Part 2: Language Interpreter and Evaluator (evaluator, interpreter) Part 3: REPL Part 4: Variables & Types (int, double, boolean, strings) Part 5: Functions and Definitions Part 6: Lists Part 7: Standard Libraries