VLM Custom Pipeline and Custom Presets

In the documentation, there is a line about custom VLM configurations, only available if admin allows it...
How is this made available? via config file? env variable?
Is there way to override the preset (with only env variables or config files)?

```
string (Vlm Pipeline Custom Config)
Custom VLM configuration including model spec and engine options. Only available if admin allows it. Must include 'model_spec' and 'engine_options'
```

If we go with non-deprecated params, you end up with this error

```
ERROR:docling_jobkit.orchestrators.local.worker:Worker 0 failed to process job 81d9b50a-0099-4e64-9a83-97ea5aca45f8: Custom VLM configuration is not allowed. Please use a preset or contact your administrator.
```

Start by locating the documentation entry for custom VLM configurations and the configuration handling for admin permissions and presets. Check how environment variables and config files affect these options, then document the supported setup and overrides, including the restriction shown in the error message.

Written by the indexing model from the issue text.