publish docs the new way

mv publish_docs_new.sh publish_docs.sh ; rm src/python/pants/docs/*.rst , other Sphinx things

the parts I only halfway-understood, listed in order of rising scariness:

Did some clean-up in our scripts:
rm'd sphinx things from 3rdparty/python/dev_only_requirements.txt . which was all of it so...
rm'd 3rdparty/python/dev_only_requirements.txt
rm'd DEV_REQUIREMENTS from pants_venv
rm'd reference to DEV_REQUIREMENTS from ./pants , use REQUIREMENTS instead.

There might have been other cleanup possible. Backed off from some possible
changes that got into gnarly territory.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/40938680

./build-support/bin/publish_docs.sh -opd sitegen-test

for the ./pants change:
export WRAPPER_REQUIREMENTS="tmp_requirements.txt" # file with one line: scrapy
in some temporarily-added pants code, from scrapy import Spider, Item, Field
worked OK. (and correctly errored out in another shell that didn't have WRAPPER_REQUIREMENTS set)

Reviewed at https://rbcommons.com/s/twitter/r/1292/
Sync of https://github.com/pantsbuild/pants/commit/8d0d75fcc11ed90f414c43ffde68817ff554a4b8