Improve parent message and documentation

I assume that DocumentJS should be able to create *something* useful out of the box. When I add DocumentJS to my project, and run it without defining a config, I get the following index page built:


It would be helpful to create a guide for configuring the `parent` and linking to it anytime DocumentJS has to guess the parent.

Run DocumentJS without a configuration to reproduce the generated index page shown in the issue. Determine where the parent configuration is documented and where DocumentJS guesses a parent, then add a guide and link to it whenever that fallback occurs; done means the guide explains configuring parent and the generated output points readers to it.

Written by the indexing model from the issue text.