use a global Composer installation

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4170

This pull request replaces all mentions of local Composer installations, but uses a gobal Composer installation instead.

**Note:** This should be considered on hold and not be merged until we have an agreement on this in #4170.
Actually, it is not. But http://getcomposer.org/download/ displays the Windows instruction only for Windows users: https://github.com/composer/getcomposer.org/blob/master/views/download.html.twig#L25