[WIP] Add high-level documentation for contributors

[charlespwd]

This PR is about adding a lot of docs to help folks understand what's going on in the codebase.

It creates the following tree of notes:

• docs/contributing/index
  • parser
  • ast
  • cst
  • ohmjs
  • stage-1-source-to-cst
  • stage-2-cst-to-ast
  • stage-3-ast-to-augmented-ast
  • printer
  • prettier
    • builders
    • what's a line, softline or hardline?
    • meaningful lack of whitespace handling
  • idempotence
  • learning-tips