Improve sphinx documentation

We used to use wiki (https://wiki.openstack.org/wiki/TaskFlow) for TaskFlow documentation, but it seems it is time to change. Let's move some of the docs to docs/source in our repository, and write some new contents there. More precisely,
- reference and examples will live in the same repository as code, and generated with sphinx when needed;
- we should keep using wiki to describe design choices, mind-blowing concepts, tips and tricks and best practices, and such things.

Key benefits I see:
- reference is kept near the code, so it is clear which version it describes; we can have different docs for different versions easily;
- reference documentation will benefit from cross-references, contents autogenerated from code and docstrings and other neat stuff;
- we'll get better contents published on docs.openstack.org (after https://bugs.launchpad.net/taskflow/+bug/1279951 solved for TaskFlow);
- docs will go through review process, which will (hopefully) ensure its high quality and timely updates.