improve preview-ability of submodule changes

[jothepro]

As a developer I want to be able to easily preview my changes. To make sure everything is fine, I want them to be as close as possible to what they will look like in production.

Currently all submodules do just have a minimal mkdocs configuration file & will render with the default mkdocs theme. This can lead to inaccuracies in the preview if I edit the documentations of just one submodule repository, without rendering the full documentation page. Certain features/plugins are not supported/enabled in the submodule mkdocs configurations.

To achieve a setup closer the the real configuration in each repository, the docs should be configured to render with the current theme from the main documentation repository by default (material for Mkdocs), and with the same plugins activated.