[DX, symfony/flex] Create folders by symfony/flex not by the seeding of .gitignore files

[kaznovac]

This is just a nitpick, but has DX impact.

The recipe's mainfest.json can state the instruction to create the placeholder folders (similarly to dockerfile/docker-compose configurations).

Having empty committed .gitignore files or having to delete them each time the recipe is reinstalled is not the best DX.