how to plug-in identity without relying on the scaffolder.

[Rick-Anderson]

Copy from aspnet/Identity#1825

These are the important bits to know if you have scaffolded the code into your app and don't want to use default identity at all UI.

Configure MVC to authorize the Identity pages that require authorization:

services.AddMvc()
   .AddRazorPages(options =>
       {
           options.AllowAreas = true;
           options.Conventions.AuthorizeAreaFolder("Identity", "/Account/Manage");
           options.Conventions.AuthorizeAreaPage("Identity", "/Account/Logout");
      });

Configure the cookie to point to the identity pages paths

services.ConfigureNamedOptions<CookieAuthenticationOptions>(options =>
{
    options.LoginPath = $"/Identity/Account/Login";
    options.LogoutPath = $"/Identity/Account/Logout";
    options.AccessDeniedPath = $"/Identity/Account/AccessDenied";
});

Regarding the fact that we rely on the scaffolder in our docs, its totally fine. The alternative is to tell you to copy paste a bunch of code in the docs into your project which is inevitably going to go out of sync. The scaffolder is the source of truth.