Put together NMODL Developer documentation (Implementation aspects)

[pramodk]

Before the end of December, it would be useful to write documentation covering key design aspects relevant to development. This could include topics such as the organization of the lexer and parsers, the rationale behind the AST design and its generation, details of various optimization passes, and some key elements of code generation.

The goal is to ensure that, after BBP, someone could still get a good understanding of certain details that may not be clear from existing papers or current documentation.