Add more documentation to help beginners

src/Command/TestCommand.php

src/Controller/LoggedInOnly.php

src/DataPersister/PlaceDataPersister.php

src/DataProvider/PlaceCollectionDataProvider.php

src/DataProvider/PlaceItemDataProvider.php

src/DependencyInjection/Configuration.php

src/Service/ExternalApi.php

src/Service/PlaceProviderInterface.php

tests/ApiTest.php

tests/Kernel.php