docs: create manage keys admin doc

[melissahenderson]

This comes out of the Rafiki work week.

The previous version of the docs had a page for "key registry". We need to determine if we:

• still need this info
• need this info in its own page, page named Manage Keys and placed in the Admin section
• need to add anything to it

We'll need to talk about this some first, because the key registry is heavily on the Open Payments side of things, and we need to figure out whether we want to duplicate information between the two doc sets, or link to the OP docs from Rafiki.

http://web.archive.org/web/20240225131259/https://rafiki.dev/concepts/open-payments/key-registry/

Note that we also have https://rafiki.dev/integration/requirements/wallet-addresses/#create-a-wallet-address-key-pair.