We've launched some new developer documentation today!
This section has more to come, specifically around the need to create block types. That's not quite done, but what is there is a comprehensive look at creating custom templates, including defaulting to a custom template in a package, overriding core block type views, including JavaScript and CSS with custom templates and more. We've also put together a detailed screencast on overriding a core block type view layer, and why you'd want to do something like that.
Additionally, we've consolidated some documentation sections in the header navigation, and cleaned up the Documentation landing page.