[Cookbook][Configuration] documentation of Apache + PHP-FPM

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #3749
How about `Apache 2 with mod_php/PHP-CGI` and we put it under the "underscore" headline. Then we do the same below: `Apache 2 with PHP-FPM`. I think it'll help since if you *only* saw `PHP-FPM`, it's not clear if we're in Apache or Nginx.