Simplify Makefile around env files

We don't need to copy defaults.local.env to local.env if we're also
layering them.
Switch to markdown for default renderer

This will be confusing since text/html is still required as the media
type and people are going to think that's what they should write. Need
https://github.com/gittip/aspen-python/issues/296.
Scrap homepage. Simple TOC everywhere.

I was feeling really constrained by the homepage. This site needs to be
about the content! I actually looked at converting it to a static Jekyll
site, but I got mired trying to install nokogiri so I could use a TOC
plugin (and that would have been a pain to deploy anyway).