First pass at autodoc support (#464)

First pass at autodoc support (#464)

* First pass at autodoc support

* Add mkautodoc requirement for docs builds

* Linting

* pip install httpx when building docs, to make it available to mkautodoc

* Fix code example in docstring slightly

* Use latest mkautodoc to resolve rendering of code snippets in docstrings

* Fill in 'Helper functions' API docs

* First pass at documenting Client

* Add autodoc for Client

* Update to mkautodoc 0.1

* Fix typos