Create sections for imported files

Some mounted locations such as adapters are not single pages but generally complete sections.

We should create the necessary folder structure with the required `_index.md` files in the existing content so that the sections are created correctly.

Example:

The openfoam adapter docs are here https://github.com/precice/openfoam-adapter/tree/develop/docs
These pages and images are to be mounted here `content/docs/adapters/openfoam`.
This still requires the `content/docs/adapters/openfoam/_index.md` to exist.

These need to be created as part of #588