Documentation uploaded/contributed files management

Problem/Motivation

There is no documentation about how uploaded/contributed files are managed by Drupal Core, specially how it handles unused and clean temporary files. Some settings are only available in the configuration (no backoffice form or doc in settings.php)

Proposed resolution

Add a documentation page.

Remaining tasks

1. Review my first draft https://www.drupal.org/docs/administering-a-drupal-site/uploaded-file-ma... →
2. Correct it, add more info
3. Place it int into the menu (Administering a Drupal at the moment but maybe there a better place?)