Lending Protocol

[oeggert]

Concept and reference docs for the lending protocol.

Docs are based off this version of the XLS-66d spec.

Reviewer suggestions to add:

• - Include repayment specifics outlined here.
• - Emphasize the configurability of the lending protocol.

[maria-robobug]

Looks good so far, just adding my initial pass of review comments.

I think there have been some recent changes in the spec that need to be reflected in the docs.