docs: use backstage.io link to backend-to-backend auth docs

There's rightfully been some confusion in various contexts about where
to find this documentation - switching to a fully qualified docs URL
should make it easier to find.

Signed-off-by: MT Lewis <mtlewis@users.noreply.github.com>

app-config.yaml

@@ -24,7 +24,8 @@ app:
 
 backend:
   # Used for enabling authentication, secret is shared by all backend plugins
-  # See backend-to-backend-auth.md in the docs for information on the format
+  # See https://backstage.io/docs/tutorials/backend-to-backend-auth for
+  # information on the format
   # auth:
   #   keys:
   #     - secret: ${BACKEND_SECRET}

packages/create-app/templates/default-app/app-config.yaml.hbs

@@ -7,7 +7,8 @@ organization:
 
 backend:
   # Used for enabling authentication, secret is shared by all backend plugins
-  # See backend-to-backend-auth.md in the docs for information on the format
+  # See https://backstage.io/docs/tutorials/backend-to-backend-auth for
+  # information on the format
   # auth:
   #   keys:
   #     - secret: ${BACKEND_SECRET}