Support dynamic navigation in main and subfolder mkdocs.yml files

While this plugin is awesome for monorepos, it does introduce the burden of having to define the complete navigation structure manually.

It would be great if functionality similar to [mkdocs-awesome-pages-plugin](https://github.com/lukasgeiter/mkdocs-awesome-pages-plugin) could be included. The point would be to allow navigation structures to be partially defined (i.e. to add `!include` directives) yet still end up with a complete navigation tree.

As for why? Well, it would allow team members to add new docs without having to also update and maintain the `mkdocs.yml` file.

Start by examining how the plugin currently processes the main and subfolder mkdocs.yml files. Compare the requested partial navigation and !include behavior with mkdocs-awesome-pages-plugin. Done means teams can add documentation without manually maintaining a complete navigation tree.

Written by the indexing model from the issue text.