Remove widget states from notebooks

[IAlibay]

I've gone ahead and cleared all the widget states from all the notebooks under examples. This reduces the total size of the doc files from 154 MB to 60 MB.

As far as I'm aware this shouldn't affect how the notebooks render, but someone please double check the RTD results.

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

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[pgbarletta]

I did the same when I added the nbval testing. They seem to accumulate with each run.

[IAlibay]

I did the same when I added the nbval testing. They seem to accumulate with each run.

they shouldn't be getting committed, I think it's more of a case that someone forgot to clear them after an update

[lilyminium]

I think this removes the ability to look interactively at NGLView widgets on rendered docs. However I think the benefits of keeping the interactivity is much outweighed by the costs of storage compared to, say, a gif.

[lilyminium]

And the huge diffs are a pain.

[lilyminium]

LGTM. Not sure how this will interact with #251 so I'll let you merge.

[lilyminium]

Actually I'll merge now so I can build off it and replace widgets with gifs.