Expand documentation for Google auth provider when using Nix without Docker

[katrinDY]

Description

After setting up Nix and Google external auth provide, user gets issue with the path /auth/callback returning a 404. The same config works with Docker. Solution is provided in a follow-up comment.

Forum link

https://forum.cortezaproject.org/t/nix-custom-build-auth-callback-path-returning-404/252/2?u=katrinyordanova

Where to add

https://github.com/cortezaproject/corteza-docs/tree/2022.9.x/src/modules/integrator-guide/pages/authentication/external

TODO

Read the discussion and add the necessary contents to the documentation