FAST: Companion cleanup (or removal)

[sruffilli]

fast/COMPANION.md content is currently obsolete.
While it can be easily fixed, we might take the chance to discuss whether to eliminate it or not.

My proposal would be to eliminate it, while refactoring the How to run this stage chapter to document the bare minimum required to deploy each stage.

[ludoo]

Oh noes, I'm trying to get issues down to zero, and a new one pops up! 🤣

Jokes aside, 👍 on the plan. We should have a hard, holistic look at all our FAST docs.

[juliocc]

+1, let's remove it and update the main docs.