Missing documentation on integrating content from subprojects

[MakisH]

In the documentation of the documentation we need... documentation on how to add... documentation from other repositories.

We currently do this for the tutorials and for the OpenFOAM adapter, but this information is missing.

We need to add such a subproject in the _config.yml (repository and subprojects) and then we need to add it in a sidebar. We also need to update the submodules, as described in the README.md.

[MakisH]

Related to #276, but we also need a guide on how to add new components to the website.
We recently had to do this for some components, such as the FMI runner: #277.