Run sphinx to generate documentation

**Role Variables**

.. zuul:rolevar:: sphinx_source_dir
   :default: doc/source

   Directory relative to zuul_work_dir that contains the Sphinx sources.

.. zuul:rolevar:: sphinx_build_dir
   :default: doc/build

   Directory relative to zuul_work_dir where build output will be put.

.. zuul:rolevar:: sphinx_builders
   :default: ['html']

   Which sphinx builders to run.

.. zuul:rolevar:: sphinx_warning_is_error

   Whether to treat sphinx build warnings as errors. Defaults to undefined
   which means to attempt to find the setting in a setup.cfg file.

.. zuul:rolevar:: zuul_work_virtualenv
   :default: ~/.venv

   Virtualenv that sphinx is installed in.

.. zuul:rolevar:: zuul_work_dir
   :default: {{ zuul.project.src_dir }}

   Directory to operate in.