message: docs: clarify role of identity and SignInPage
diff: @@ -35,6 +35,13 @@ full list of providers, see the
  ### Identity - WIP
  +> NOTE: Identity management and the `SignInPage` in Backstage is NOT a method
  +> for blocking access for unauthorized users, that either requires additional
  +> backend implementation or a separate service like Google's Identity-Aware
  +> Proxy. The identity system only serves to provide a personalized experience
  +> and access to a Backstage Identity Token, which can be passed to backend
  +> plugins.
  +
  Identity management is still work in progress, but there are already a couple of
  pieces in place that can be used.
  