Move documentation back to Read the Docs

[janette]

Set up

requirements for local build

pip install python
pip install sphinx
pip install sphinx_rtd_theme
pip install sphinxcontrib-phpdomain

QA Steps

git checkout WCMS-13915
cd docs
make html

View the docs locally at [workspace]/dkan/dkan/docs/build/html/index.html

Resources
https://www.sphinx-doc.org/en/master/usage/restructuredtext/index.html
https://docs.readthedocs.io/en/stable/intro/getting-started-with-sphinx.html