move the publish-extras-task doc to its own page. mention repo in register.py

moved some info around in hopes folks can find it more easily
mention repo-in-register.py

before:
setup_repo had some info about setting up publishing, but the publish page didn't link to it.
setup_repo didn't mention something new and important: setting up a repo in a plugin/register.py
publish page had a section about setting up an extras-with-publish task

after:
link to setup_repo's publish section from the publish page
mention repo-in-register.py in setup_repo
mv 'extras-with-publish task' to its own page, linked from setup_repo

Bugs closed: 602

Reviewed at https://rbcommons.com/s/twitter/r/1090/
Sync of https://github.com/pantsbuild/pants/commit/7864311c4e548345ee6a5ee59f1a14ff5948340f