docs: update Peers page

[melissahenderson]

Based on some internal and external feedback, we should flesh out the Peers page a bit more.

• Instead of using lists in the Prerequisites section we should try to use sections.
• Look into adding context on incoming auth tokens. There's some info about them in the variables section. Does the context change depending on the use case? Eg., peering with the test network, peering with external Rafiki instances, peering with internal Rafiki instances. (Feedback from Collins: It's unclear what the HTTP.incoming.authTokens is for.)

/integration/requirements/peers