Pin nbsphinx to 0.8.6, clean up orphaned resources

[kevin-bates]

It turns out that the documentation for Notebook has not been properly producing its Notebook Examples section of its docs. In looking into the issue, it can be reproduced by simply using nbsphinx 0.8.7 (over 0.8.6). However, I suspect there may be more to this than that, but this is a start. This pull request pins nbsphinx to 0.8.6.

While looking into this I noticed the following:

1. A warning message was produced that docs/source/examples/Notebook/header.md was not listed in any toctree - which is true. However, the contents of that file are already equivalently expressed in docs/source/examples/Notebook/examples_index.rst, so I've removed header.md.
2. The "Contributing to the Jupyter Notebook" section is a straight include of CONTRIBUTING.rst at the repo root. In that file, a simple verification image is referenced down in docs/source/_static/images. However, while that image will be rendered when viewed from GitHub, the same link is invalid relative to the generated docs. Since the image didn't really provide much information beyond what the text already describes, I chose to remove this image altogether, thereby removing the erroneous link produced in the ReadTheDocs version.

Resolves #6193

[kevin-bates]

Could one of the existing RTD maintainers look at the RTD builds and determine when a second build is always triggered and always fails as a duplicate? If I rebuild the failing build it succeeds - as does the first of the two builds.

Looking at the docs produced, I see all the examples - indicating this PR will resolve the issue, yet the duplicate build failure is preventing (I think) RTD from deeming the build successful.

I've also gone ahead and "activated" the latest release-specific build - having noticed this hasn't happened since 5.x.

cc: @willingc, @blink1073, @Carreau (Sorry you three, but I figured you'd be most likely to respond. 😄 )

[blink1073]

I think it is due to the multiple webhooks set up: https://readthedocs.org/dashboard/jupyter-notebook/integrations/

[kevin-bates]

Interesting - that seems like a good bet. Looking at the hooks' links, the first appears to get triggered monthly, while the 2nd and third are more frequent (and similar). Can you determine if both the 2nd and 3rd are necessary or how they differ?

[blink1073]

I'd say remove either 2 or 3, since both appear to be tied to PR builds. That should clear up the duplicate builds. Then, create a test PR and make sure the RTD build is still fired.

[kevin-bates]

Thanks Steve. I've deleted the 2nd webhook and pushed an innocuous change to this PR (hoping this is sufficient to trigger RTD).

[blink1073]

Seems to be working!

[blink1073]

(and no duplicate build this time)

[kevin-bates]

Right on. The generated doc looks correct as well (relative to this PR).

[willingc]

Thanks @kevin-bates I'm going to merge this PR since the Notebook Examples are displaying on RTD now. Thanks @blink1073 too. The JS test failures appear to have existed for a while and are not related to this PR.