Match navbar with dask-sphinx-theme

[mrocklin]

The dask.org navbar has different content, and is somewhat less pleasant than the navbar used in dask_sphinx_theme (every other dask page). It would be good to move the sphinx theme's navbar over to the main page in this repository.

I would be happy with a copy-paste job here, but ideally we would find some way to have this repository mirror changes in dask-sphinx-theme.

Does anyone have any suggestions on if this is possible, and if so how?

cc @TomAugspurger @dsludwig