Documentation of the overall language

@michaelaburr - Michael Burr suggests additional introductory material about the Macaulay2 language be added to the documentation to make it easier for newcomers, especially for those knowing C:

• an introductory section about the overall syntactical structure of the language
  • everything is an expression, there are no statements
  • list processing is important; there are also hash tables
  • the class hierarchy is present and important
  • method functions versus compiled functions
  • classes don't own methods, methods for binary operators are stored in the youngest type
  • all objects are on an equal footing (everything has a hash code and can be used
    as a key in a hash table) (everything has a class and a parent)
  • some operators are mathematical and some are oriented toward low-level tasks
• in the section on classes
  • how to explore existing objects in the system (peek, peek')