Update Authentication page with Google SSO instructions

[The Authentication page](https://admin.forem.com/docs/advanced-customization/config/authentication) is now missing Google SSO instructions for admins to be able to configure Google SSO on their Forems. 

[Here is the announcement post on forem.dev](https://forem.dev/foremteam/changelog-new-year-new-user-authentication-provider-sign-in-with-google-3gk7)