document how to configure services

Hello,

First of all, thank you very much for this project.
It is amazing to have so many services available to easily configure in Process Compose.
Thank you very much.

I would like to request that it be documented how to configure the different services, I mean, literal config options.

For a reference, this is all you see in the postgres docs:

```
# In `perSystem.process-compose.`
{
services.postgres."pg1".enable = true;
}
```

For example, today I had to browse the postgres service definition to see that it accepts a dictionary of values called Settings, where I can put the configuration I want. This is very convenient and I have been going around and making all kind of weird attempts in order to be able to try to configure it correctly when I could have done it from the beginning simply by providing this argset.
It would be amazing to have it documented upfront.
Thank you very much.
It will be useful even if this was to simply say `you can put settings into settings key` and then link to the official documentation of the options for this service.