Allow PHP XDebug use with the Docker dev server
Added Stripe payments & books download page

* added a "Stripe payment manager" Service
* added a BookPayment DB Entity
* added a "download book" page
* added a "alina_reyes_ebooks_store.books.files_formats" parameter
* added the "check bought books access" implementation in our "books access checker" Service
* added a "homepage" lik to the site title
Removed "minimum-stability: dev" in composer.json