Flatten documentation root for single-module documentation

[ParadoxPixel]

Currently every documentation website includes an index page which functions as the root of the documentation listing all documented modules and providing a nice entry into them. For single-module documentation it would however be prefered to not have this index and rather step into the single module directly. This results in a nicer user experience as it skips an unnecessary step in navigating the documentation.

The expected behaviour would be:

1. In case of multiple modules keep the index page and navigation tree as is.
2. In case of a single module flatten the documentation to have the only module be the root.