Please hold... I'd like to add improvements to this PR.

I need to clarify a few things:

1. Can encore be installed in three ways? (i.e., using yarn, npm, or composer)
2. Or is encore installed by either npm or yarn only, then one has to run composer require encore?

The reason I asked for the latter is due to the fact that installing encore using composer doesn't create the expected node_modules/ directory. Without this directory, a newbie like me won't be able to follow the next set of instructions.

This should be ready for review. I figured that the composer package encore is just there to initialize encore for your Symfony project. But the actual installation is through Yarn or Npm.

Closing in favor of the identical #8674, because I noticed it first ;).

@mahalay we only need the one PR, we'll merge into all branches.