Update & move key registry documentation

[mkurapov]

Context

The current key registry doc should live in the Open Payments site. Instead of this document, we should have a document explaining what it means for an ASE admin to add a key to a payment pointer.

[melissahenderson]

@mkurapov

we should have a document explaining what it means for an ASE admin to add a key to a payment pointer.

Does this address your request and, if so, is there anything else we need to do on the Rafiki side? https://rafiki.dev/integration/requirements/wallet-addresses/#create-a-wallet-address-key-pair

I have the content from the old Rafiki page and will check that the appropriate bits are in the Open Payments docs.

[mkurapov]

@melissahenderson this is good 👍 I think we can close this issue