Fixing some problems in the documentation:
- Better explain the default layouts used by pages and posts.
- Downgrade `site/root` to something people wouldn't change in most cases, and
  don't use it in examples anymore.
- `pagination` properties for next/previous page URLs can be used without `pcurl`.
- Page config settings are under `page`.
- Correct usage for `--root` parameter in `chef`.
- Added dedicated page about config variants in the "Advanced" section.