Fix doc build (a bit)

[jlmelville]

Me again. Yes, I am on a tear today.

When sphinx 1.8 came out (back in, er, 2018), the api changed add_javascript to add_js_file. I have updated the conf.py to reflect this and set a minimum requirement in docs_requirements.txt (although I can't imagine the latter being an actual problem).

Now I get a fair number of errors when I run sphinx-build but they are of the fixable type rather than of the oops-sphinx-exploded type.

Also I added a section to the CONTRIBUTING.MD about how to build the docs locally. I set the build output to that specified in doc/Makefile (which is _build) although I couldn't get make or make help to actually do anything.

Finally, I added the build output to .gitignore.

[lmcinnes]

Thanks for this. I admit I haven't been paying attention to the doc builds recently, so noting that sphinx changed and it got broken is greatly appreciated.