Prepare documentation for readthedocs

Create Makefile and recreate conf.py using sphinx-quickstart
Add _build to gitignore
Fix several nits in rst files
Add tox environment for docs building. Now docs may be built by
running `tox -edoc'

Change-Id: Ib0eb17047e583bdf3ac9111e117f908340f4d5ef
Merge "Prepare documentation for readthedocs"