Move repository to organization

[marein]

This repository should be moved to https://github.com/gaming-platform and the main application Docker image should be made accessible via ghcr.io.

Reason
Everything else about the project is already or will end up in the organization. There is no reason to leave this repository in my personal account.

Naming
The name php-gaming-website will change. Since this is a modular monolith, this will not match with the current repository naming conventions like gaming-platform/service-js-polyfill, gaming-platform/service-mailer, gaming-platform/docker-php-fpm or gaming-platform/php-mailer-client. It could be named to gaming-platform/application.