[Documentation]: improve docs around IdP integration

[sabineschaller]

Right now, we only have a short paragraph about the IdP in the Getting Started docs. They also reference some docs in the auth package, but there is no link.

Additionally, we explain the interaction flow here. However, we don't link to it from the Getting Started page.

Finally, it is not clear from the Interaction Flow docs what the id is. Since the route is /grant/:id/:nonce, one may think it's the grantId. However, it is the interactId. This needs to be clarified.

Long story short, we need to completely overhaul our docs on the IdP and how Rafiki interacts with it.