[12.x] Allow sub-array notation in arrays validation

[fgaroby]

This PR allows to use the sub-array notation, in addition to the dot notation.
For example, instead of this:

class NestedArrayRequest extends FormRequest { public function rules() { return [ 'nested.*.bar' => 'required', 'nested.*.baz' => 'required' ]; } }

We can now do this:

class SubArrayRequest extends FormRequest { public function rules() { return [ 'nested' => [ '*' => [ 'bar' => 'required', 'baz' => 'required', ], ], ]; } }

Usefull when the sub-array has many sub-fields.

Of course, the nested notation still works.

If this PR is OK, I also created a PR to improve the Laravel documentation.

[shaedrich]

I already attempted something similar a while ago in #55491, so don't get your hopes up. That being said, it'd be nice to have this in Laravel, so 🤞🏻