docs: update "Overview and checklist" page in Integration > Requirements

[melissahenderson]

This issue covers a number of updates to the Overview and Checklist page. We should wait until #3166 is merged before working on this.

Expose an exchange rate endpoint
Change the first sentence from "To support cross-currency transaction" to "If you plan to support cross-currency transactions". I think this makes it more obvious that it is optional.

Charge a sending fee

• Change section name to match the corresponding link ("Define your sending fees").
• Change the first sentence from "You can charge a sending fee" to "You have the option to charge sending fees". Same reasoning as above. It's a bit more wordy, but I think it makes it more obvious that it is optional.

Integrate with an identity provider (IdP)
Replace the second paragraph with this: Integration with an IdP is required if you plan to use the authorization server provided through Rafiki's auth service. The authorization server requires consent to be collected via an interactive grant before an outgoing payment request is issued. The purpose of the IdP is to handle the authentication and consent required to authorize the interactive grant request.

Peering
If not included as part of #3166, the Overview and Checklist page should contain a section for peering. The section should briefly explain when peering is required and link to the Integration > Requirements > Peers page.

For all conditionally optional items
We should provide some kind of visual to distinguish between what's required and what's conditionally optional. Two suggestions are to separate them into different sections or use badges. I'm open to other ideas.

Integration checklist
After making the above updates, review the integration checklist and make any necessary adjustments. Also ensure an item for peering is included.