Document deferred using sphinxcontrib-emacs

This is the PR to track migration of deferred documentation to sphinxcontrib-emacs
https://github.com/flycheck/sphinxcontrib-emacs

TODO before pull:
- [ ] Setup readthedocs
- [ ] Wait until markup of sphinxcontrib-emacs is settled (probably until it is relased?:  https://github.com/flycheck/sphinxcontrib-emacs/issues/3)
- [ ] Copy document README.markdown to docstrings in deferred.el and concurrent.el.