gatsby-theme-ocular: Simplify/document what is required to override templates.

[ibgreen]

Copying templates (like templates/documentation.jsx) and overriding them with gatsby-config options still requires some fix ups in the copied file, maybe we could add some comments/documentation on how exactly to do that?

1. Ideally, we change the templates to not use local imports. If that is done the imports won't have to be changed when copying.
2. I believe the duplicated gatsby query error can be ignored (as both queries are identical). But this should at least be explained in the docs.