docs: OP grant revocation and other OP info

[melissahenderson]

Creating this issue based on feedback the team received.

Update Rafiki docs to ensure grant revocation and other Open Payments-related information is well-documented and logically placed.

Background:
Revoking a grant is not driven by the ASE. It’s something a user does. The ability to revoke a grant is functionality the integration should/could provide to a user. We should document this for the Rafiki docs. Example for more context: in our test wallet, a user can view all the grants they’ve approved, with the option to revoke them. Viewing and revoking grants isn’t a hard requirement for an integration itself, but is strongly suggested. One suggestion provided by Max is: “Maybe under Requirements, have an ‘Open Payments support’ page under which we have the IdP, grant revoking, and potentially moving the wallet address keys management into."