Switching to readthedocs Sphinx template

[philippjfr]

I don't like the default style we use across ioam packages very much and think the readthedocs template would be a much better alternative. I've mocked up what this would look like here. It's pretty trivial to switch over but it requires writing proper toctrees, which has not been consistently across ioam packages. I'd suggest switching over this package now and if Jim finds some time to write toctrees for topographica we can switch all the other packages as well.

[sf-issues]

I don't really see the advantage of the readthedocs template, but if you want to do the work of switching, it's fine by me.

[philippjfr]

Recently switched, closing issue.