Write tests to validate reference docs are complete

[LiamConnors]

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