When using split-by-submodule, documentation for the root of the project is not generated

[mbonig]

Currently using jsii-docgen with projen and would like to have docs split up by submodule, using the --split-by-submodule option. However, when using this, no documentation is produced for the constructs in the root of the project.

The full command:

$  npx jsii-docgen .jsii -o docusaurus/docs/api/api --split-by-submodule

With this, I get an 'api.md' file in the docusaurus/docs/api directory that has the following contents:

# Submodules <a name="Submodules" id="submodules"></a>

The following submodules are available:
- [awscdk](./awscdk.md)
- [build](./build.md)
- [cdk](./cdk.md)
- [cdk8s](./cdk8s.md)
- [cdktf](./cdktf.md)
- [circleci](./circleci.md)
- [github](./github.md)
- [gitlab](./gitlab.md)
- [java](./java.md)
- [javascript](./javascript.md)
- [python](./python.md)
- [release](./release.md)
- [typescript](./typescript.md)
- [vscode](./vscode.md)
- [web](./web.md)

But none of those files contain any of the documentation for files in the 'src' directory, like gitpod.ts, ignore-file.ts, and others.

How can I have smaller, more fine-grained documentation files, without losing documentation for constructs in the root of the project?