Drop the navbar

The current navbar has links to pages that I think mostly don't matter.

- distributed docs are way too technical for most users
- dask-ml is not actually all that focused on dask + ml users do today
- examples is ok, but doesn't get great engagement
- ecosystem a page with a bunch of links on it
- community is intended well, but doesn't seem to actually get people to do community things

The only link I actually think is highly valuable there is the "Dask" link, which will bring people back to the main docs if they're on a subproject site (like dask-jobqueue). That could probably be managed through an image/link on the top of the left-sidebar.

I'm also open to new links if people have ideas. If we can't find a good list though then I think it's good to focus peoples' attention on the TOC.

cc @scharlottej13 @jacobtomlinson @jrbourbeau