setup_repo: now only 98% impossible

This page was full of doom and gloom: Want to start using Pants? Might as well just give up.

Walk back some of that. Point out some files that you need to copy+adjust.

This probably still doesn't capture everything folks need, but it could be a step in the right direction.

rm the howto_customize page. For a while, I thought such a page would be useful... but its material all falls into setup_repo and howto_plugin.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/40251344

Bugs closed: 632, 726, 776

Reviewed at https://rbcommons.com/s/twitter/r/1295/
Sync of https://github.com/pantsbuild/pants/commit/0bf618b78e743358735555244819de0751c295ad