[DX] Add contrib and custom directories to the docroot/modules directory.

Add contrib and custom directories to the docroot/modules directory. How do people feel about this? We added a files directory for easier site installation.

I personally strongly dislike when I inherit a site and the contrib and custom modules are all mixed together.

We could make it easy for the site builders and developers to use best practices by having this structure in place out of the box with README.md files in each explaining what types of modules belong in each.

Are there reasons we should not do this?