Show on mobile when a document has sub docs

[virgile-dev]

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe.

it's not easy to realize on mobile that the document root has children.

Describe the solution you'd like

@rl-83 suggests a simple first solution for mobile-only to convey this information (see screenshot).

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Discovery, Documentation, Adoption, Migration Strategy
If you can, explain how users will be able to use this and possibly write out some documentation (if applicable).
Maybe add a screenshot or design?

Do you want to work on it through a Pull Request?