chore(docs): Add some todos

docs/authentication.mdx

@@ -1 +1,5 @@
 # Authentication
+
+{/* TODO: explain internal authn vs delegated authn */}
+
+{/* TODO: consider explaining UIA authz versus OAuth authz here */}

docs/authentication/oidc.mdx

@@ -0,0 +1,3 @@
+# OIDC delegated authentication
+
+{/* TODO: copy stuff over from guides/oidc.mdx and refactor */}

docs/authentication/registration.mdx

@@ -35,3 +35,5 @@ All email options are in the `[global.smtp]` block of the configuration file, an
 connection_uri = "smtps://john%40example.com:password@mail.example.net:465"
 sender = "John's Continuwuity server <john@example.com>"
 ```
+
+{/* TODO: Add relevant admin commands for email */}