Add an extra script in `Contributing` to install some of the extra `Doc` requirements i.e. to make it easier to dev & QA

[archmoj]

Using these commands from Contributing

(plotly_dev) $ pip install -r packages/python/plotly/requirements.txt
(plotly_dev) $ pip install -r packages/python/plotly/optional-requirements.txt

won't install modules e.g. wget, datashader, etc. that are used in docs. I suggest we add optional-doc-requirements.txt to help install some of these modules so that we could more efficiently test the examples using:

jupyter-notebook doc/python