doc: move badges to the end of the readme and remove landscape badge

Due to snakeoil's widespread use of demandloaded modules the lint scans
done by landscape.io aren't very helpful without using the custom lint
plugin provided along with snakeoil.
doc: use sphinx-apidoc to auto-generate api documentation

This also drops some of the snakeoil.test API from being documented, but
realistically the only consumer of that should probably be pkgcore and
related tools.