document the usage of PHP-FPM on Apache
feature #3915 [Cookbook][Configuration] documentation of Apache + PHP-FPM (xabbuh)

This PR was merged into the 2.3 branch.

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

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #3749

Commits
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822b3bd document the usage of PHP-FPM on Apache