Changes to how we build the docs

[rjleveque]

I'd like to propose the following:

1. Switch from using sphinxcontrib-versioning to sphinx-multiversion as discussed in #209.

2. Stop using nbsphinx to generate doc files directly from notebooks. We are not really using this currently anyway, and rebuilding some of the old docs is now failing because, for example, there used to be a doc/pyclaw/tutorials/burgers/pyclaw_tutorial.ipynb file that no longer exists, but when trying to build v5.3.1, for example, it tries to run nbconvert on this notebook, which fails since this doesn't run with the current clawpack.

   Using nbsphinx also means making the docs is slow if it has to do this for all past versions each time.

   I think the more robust system, which I've already been using, is to keep the tutorial notebooks we want in $CLAW/apps/notebooks and use the script gallery/notebooks/make_html_from_notebooks.py to convert them to html version in the gallery of notebooks.

3. Stop using intersphinx to link to specific gallery pages from the main docs. This is very lightly used and often doesn't seem to work right anyway. Since we're not versioning the gallery and it's just a static set of pages, I think from the main docs we can just like to a specific page like http://www.clawpack.org/gallery/notebooks.html where desired.

I've been having problems recently with sphinx getting into a semi-infinite loop creating directories like _build/html/_build/html/_build.... that seems to be a result of some combination of the above tools.

[ketch]

All of these changes seem good to me.

[mandli]

+1

[rjleveque]

OK, I will implement all this for v5.7.0.