doc: publish page shows provides= example

We have some lovely sample code for provides=; but folks overlook it. So point folks
at it, sheesh.

setup_repo.md: replace copy-paste sample code with an include, lest it drift out of date without us noticing

Testing Done:
see it rendered at
http://pantsbuild.github.io/lahosken-demo/publish.html
http://pantsbuild.github.io/lahosken-demo/setup_repo.html#setup_publish

Bugs closed: 473

Reviewed at https://rbcommons.com/s/twitter/r/1549/
Sync of https://github.com/pantsbuild/pants/commit/6f1d2a516bd0220ca71cea0a76742c237ea2469d