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>

Author: jasnell

.eslintrc.js

@@ -54,6 +54,7 @@ module.exports = {
     {
       files: [
         'doc/api/esm.md',
+        'doc/api/fs.md',
         'doc/api/module.md',
         'doc/api/modules.md',
         'doc/api/packages.md',