doc: refactor fs docs structure

* Convert examples to esm to help promote that pattern with users
* Make Promises APIs more prominent to help promite that pattern
* Separate callback/sync apis into distinct sections to make those
  more consistent with the Promises api
* Improve other bits and pieces

Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: nodejs#37170
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>