docs: Add page describing atom arguments and results

Task Arguments and Results page from wiki converted to sphinx/rst
and edited to improve readability and make it look better. Some
examples are converted to doctest.

We also now check doctests from docs for correctness when unit
tests are run by tox.

Partially implements blueprint sphinx-docs.

Change-Id: Ief398856e1ce2d0dc1d43c4e4112872a27672fcf
Merge "docs: Add page describing atom arguments and results"