Rearrange, update, extend docs

[Technologicat]

• In the README, have a quick overview.
• Have the main level of detailed docs in a separate file.
  • Keep the separate docs for quasiquotes, REPL.
  • The walker docs could go into the main detailed doc, it's just one class.
• Move all docs except README to a doc/ subfolder.
• Add more code examples.
  • Show how to use step_expansion, and example output. Important feature.
• Write a separate doc for the dialect subsystem to help discoverability. Material already exists in docstrings.
• Write a short what can I do with macros? section for new users.
  • CAUTION: Macros are the 'nuclear option' of software development.
  • Often a good strategy is to implement as much as regular functions as reasonably possible, and then a small macro on top, for the parts that would not be possible (or would be overly verbose, or hard to follow) otherwise.
    • unpythonic.test.fixtures is an example, as are let constructs (though in that case the macros are rather complex, to provide lexical scoping).
  • Extract patterns that cannot be extracted as regular run-time functions, hence eliminating boilerplate.
  • Control the evaluation order, and whether a particular expression or statement runs at all.
  • Implement language-level features inspired by other programming languages (expression-local variables, auto-TCO, autocurry, lazy functions, multi-shot continuations).
  • Beside productivity benefits, useful for teaching and learning. To understand is to implement.
  • Macros not always needed for this. Pattern matching. Resumable exceptions. Multiple dispatch.
  • Examples from Seibel, Graham, unpythonic, and the Racket community [1] [2] [3].
  • Anything useful from my earlier notes on this topic.

[Technologicat]

Mostly done now already for 3.0.0. Remaining TODOs:

• In the README, have a quick overview.
• Have the main level of detailed docs in a separate file.
• Add more code examples.
  • Show how to use step_expansion, and example output (colors and all). Important feature.

[Technologicat]

README separation finished in 421996d.

Now just needs more examples. Maybe actually leaving that until 3.1.0.

[Technologicat]

Since the rest of this is done, moved the needs more examples item to a new issue, #14.