docs: update requirements overview page

[melissahenderson]

The Peer section on the overview page needs a little work.

• Change the title of the "Peers" section to be parallel with the other sections (sentence starting w/ a verb). Example "Add a peer to enable Interledger payments"
• Decide whether we should continue to show it as required, or change it to "conditionally optional" even "conditionally required" in orange (it was kind of a joke, but also kind of not).

/integration/requirements/overview