Add rake task to generate default config doc

Continuation of #2452. Let me know what you think!

Some changes from the original documentation:
- `destination`, `plugins`, and `layouts` began with `./`, but now those are stripped.
- `encoding`, `relative_permalinks`, and `paginate_path` now have the correct values (they were not up to date before)
- `url` does not exist anymore (I don't think Jekyll used this)
- `watch`, `server`, and `paginate` are the deprecated/common issues. Those are removed.
- Spacing
- Order