The auth backend code now follows the same registration pattern used

throughout the codebase. Auth backends can be registered, unregistered,

iterated through, and looked up.
A new requirement is that auth backends must have an ID. For

compatibility purposes, we assign an ID if one is not set when scanning

entry points, but all registrations going straight through the new

functions will require an ID set directly on the class.
There's also a new AuthBackendHook for extensions, which will handle the

registration and unregistration for the extension.
Documentation has been added for the new hooks, and the guide on writing

an auth backend has been updated as well.

Made use of this in an extension. When I enabled the extension, the auth

backend appeared in the list, and could be used. I was able to authenticate

with it. Disabling the extension removed it from the list.
Built the docs.
Checked that the right backends were loading and being provided to templates.