Feature: generate sphinx documentation

[ronaldoussoren]

I'm working on a "docgen" subcommand that will generate sphinx documentation.

• Add "docgen" subcommand
• Document "enum" values and types
• Document "externs" values and types
• Document classes and methods (mostly done, but can be better)
• Document functions
• Document aliases
• Document expressions
• Document literals
• Implement a mechanism to add introduction text to the file (pre-amble)
• Implement a mechanism to document additional methods (such as convenience methods, but also custom __new__)
• Cross-module references need work