Try `show_submodules: true` in all modules

Would something like result in not having to specify the submodules manually?
```md
::: pyrenew
    options:
       show_submodules: true
```

for example, in https://github.com/CDCgov/PyRenew/blob/main/docs/reference/latent.md we have 

```md
# Latent

::: pyrenew.latent

## Temporal Processes

::: pyrenew.latent.temporal_processes
```