Add a Highlights section to the 4.0 release notes.
Flatten doc navigation

sphinx_rtd_theme's sidebar will not go deeper than two levels
in the table of contents, so remove the extra index->documentation
indirection.
Reorganize top-level doc navigation.
Drop the 3.2-era "upgrade notes" from the homepage.

The need to install dependencies is well-established by now.
Move next.rst to 4.0.0.rst
Set version number to 4.0