docs: mention mvn2pants

while I was in there: setup_repo had a paragraph about top-dir BUILD files for config. LOL, nowadays, that config has moved into plugins. So pull that out, and re-jigger examples of BUILD.foo files.

Reviewed at https://rbcommons.com/s/twitter/r/1301/
Sync of https://github.com/pantsbuild/pants/commit/98f762d671f11425fa291035fa330e5beb304835