refactor config wizard

[pmeier]

This is my proposed solution for #124. It implements the following changes:

• Remove the demo and builtin special configuration options. Note that the previous Config.demo() is still available, since that just created a Config() with default values anyway:
  

  ragna/ragna/core/_config.py

  Lines 123 to 125 in a04efc2

  	@classmethod
  	def demo(cls) -> ragna.Config:
  	return cls()

• Remove the default value for -c / --config. ragna worker, ragna api, and ragna ui now require passing a configuration.
• Split ragna config and ragna config --check into ragna init and ragna check. ragna init is the configuration wizard and ragna check can be with configuration files to check availability of the requirements.
• Improve the wizard by printing a summary of the unmet requirements as well as instructions on how to meet them.
• A few phrasing improvements in the wizard.

[pmeier]

One more change after an offline chat with @dharhas:

Instead of asking for confirmation after every single component that has unmet requirements, just assume that is what the user wants, since they have selected this in the step before. Since this PR also adds a summary of all the required packages and environment variables at the end, no information is lost through this.

We also discussed to have a "I just want everything that ragna has to offer" option, but I decided against it for now. Due to the change above, the selection is now only two questions. Coupled with the fact that you can toggle all options by pressing the a key when the choices are visible, IMO, we would make the process more complicated by adding another question. LMK if you feel different.

[smeragoel]

Agreed, simpler is better. Two options should be fine, and instead of over-optimising now, we can wait for user feedback to iterate.

[pmeier]

One more final adjustment after another offline chat with @dharhas:

We now use the config file located at ./ragna.toml as default in case it exists. Meaning, if someone generates a config with ragna init, all the other commands work without explicitly setting the -c / --config option.

[pmeier]

Doc error is unrelated:

ERROR - mkdocstrings: Couldn't load inventory {'url': 'https://fastapi.tiangolo.com/objects.inv'} through PythonHandler.load_inventory: HTTP Error 500: Internal Server Error

The FastAPI site is down.