How to add in Drupal 8 a custom (suggestion) default contact page template ?
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Way 1: Use drupal template suggestion:
You can enable the template suggestions by following changes as mention in https://www.drupal.org/docs/8/theming/twig/debugging-twig-templates
Once you have the suggestions enabled you can see, drupal suggests few template possiblities for any page, you can use any of the suggested template.
Way2: Declare custom template suggestions
Or you can customd define template based on any logic by implmenting