add: support for rubyfmt as ruby fixer (#4678)

[rubyfmt](https://github.com/fables-tales/rubyfmt) is a formatter for
 `ruby` code.

This commit adds support for `rubyfmt` as a `ruby` fixer (#2991),
together with some tests and documentation.

autoload/ale/fix/registry.vim

@@ -641,6 +641,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['nickel'],
 \       'description': 'Fix nickel files with nickel format',
 \   },
+\   'rubyfmt': {
+\       'function': 'ale#fixers#rubyfmt#Fix',
+\       'suggested_filetypes': ['ruby'],
+\       'description': 'A formatter for Ruby source code',
+\   },
 \}
 
 " Reset the function registry to the default entries.