Remove numbers from URLs.

This will let us keep docs in order on the filesystem but have
reasonable permalinks. If we add something at 02- it doesn't result in
lots of broken links.
Autogen the docs nav

This is still the same amount of duplication, except way easier to
parse. Docs nav now lives in `_config.yml` and you must restart jekyll
to see changes to that list (since config is only read at jekyll launch)