Documentation updates

[drasmuss]

Switches the documentation to the new theme, and adds versioning to the build process.

[tbekolay]

I think this is also something we should have in 0.4.0. I looked it over and I think this is basically good to go, except that we should add in the Nengo Loihi logo from nengo/design#7 as currently there's no other way to get back to the index page. Aside from that, is there anything else you want to do in this branch @drasmuss ?

[tbekolay]

Also a heads up that I rebased this!

[drasmuss]

Pushed some fixups, and added a logo and favicon from nengo/design#7. Took a look at the built docs and they look good to me!

[drasmuss]

Added a fixup to change the logo and adjust the width (with the new nengo-sphinx-theme release).

[tbekolay]

Pushed some quick fixups for the latest nengo-sphinx-theme release, will merge if you're happy with it now @drasmuss

[drasmuss]

Looks good to me. I added a commit to apply the patch from nengo/nengo-bones#3. Not strictly necessary since we don't allow PR builds in this repo, but doesn't hurt.