Papermill: Update papermill docs

[MSeal]

This is mostly a self-reminder as prep for Jupyter Con. We've made a lot of improvements to papermill this year -- some of them aren't transcribed to the docs well yet.

[willingc]

I've set up a webhook for RTD to autobuild on pushes in GitHub. Send me your RTD account name and I'll add you to the maintainer list too.

[willingc]

Any particular features or sections? If so, we can open a few doc issues for new contributors.

[MSeal]

Thanks for setting that up! The updated API docs and developer_guide help a lot here with docs now.

I think a better parameters section that described options for passing to the notebook would be beneficial. I get a lot of questions about that.

And a how-to-tag section for different notebook interfaces would help users.

I'll add these two as separate new contributor issues.