chore: Simplifying models management and verification on initial bootup

[LuchoTurtle]

As it stands, even though the models are cached after an initial download, managing different models is a bit hard and confusing.
You can't use two concurrent models, one for testing and another for production, at the same time.

This is not optimal, especially while developing. If we wanted to run mix phx.server and mix c to alternate between testing and implementing, I am forced to delete the models and re-download them (because we are using different models for testing and production scenarios).

Therefore:

• Remove the need to use BUMBLEBEE_CACHE_DIR through env variables.
• Simplifying config.exs, prod.exs and test.exs.
• Make changes to Dockerfile so it doesn't need to use BUMBLEBEE_CACHE_DIR.
• Make models download to their own folder and cache them accordingly.
• Document and deploy.