Remove the special demo and builtin option for the config?

[pmeier]

The original intention was to make it easier for the user to select these two without the need of a config file. However, it is beginning to dawn on me that this actually makes it more confusing:

By having builtin as default, starting the worker / API / UI without a config is possible. However, we are only including components for which the requirements are met. So if one does pip install ragna[builtin], they would only see the builtin source storages, since all assistants so far require an API key to be set as environment variable. Worse, if one does pip install ragna, the default builtin configuration is the same as demo, since now we likely not even meet the package requirements.

By removing the demo and builtin option and in turn require the -c / --config option, we would "force" users through the config wizard. And by that they should have a better understand what they are running.

[smeragoel]

I think that makes sense, since we want them to go through the config to get a better understanding.

[pmeier]

Closed in #125.