Write tests to validate reference docs are complete

Write tests to validate that the reference docs are complete and ensure they stay up to date as changes happen in Plotly.py and Plotly.js

Plotly.py mkdocs branch is here https://github.com/plotly/plotly.py/tree/api-docs

Plotly.js docs to be implemented here https://github.com/plotly/graphing-library-docs/issues/346