Better documentation template management

[sstenchlak]

There should be an easy way to define a template, or part of a template, as a separate model. For example, a template for an organization-specific chapter.

There should be a mechanism to easily override defined templates in a hierarchical manner. Each package should be able to redefine the template for its sub-packages. This needs to be further examined as this "overriding" is useful for configuration and other models as well.