Make the blog module easier to understand for new devs or document it better

An onboarding problem with the blog module:

Currently there is no `body` schema field for a blog post, because we are assuming people will use `contextual: true` and know how to set that up. They probably don't. The generic "show" template has no body, so you don't really get a blog "out of the box" with this module.

We have to document this better if adding body to the schema is too much of a risk bc-wise, which I suppose it is. We could at least add it to "show.html" and document that you are expected to either use "contextual: true" or add a body field.

cc @colpanik @stuartromanek